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Mineralogy and Petrology Notes
Go over syllabus
Review rock cycle, types of rocks, how they each form and the story that they tell.
Play game: Name that rock or mineral
1 point for each mineral identified, 1 point for composition
1 point for each rock identified, 1 point for story it tells.
Physical Properties of Minerals
Crystal faces: faces are planes in the crystal with particular ion/atom densities and
arrangements.
Faces reflect underlying symmetry of the crystal
Habit: Malformations, differential growth rates, restrictions of growth area
euhedral
subhedral
anhedral
Luster, color, streak
luster: way light reflected, refracted, metallic and nonmetallic: vitreous (Qz), resinous
(sphalerite), pearly (talc), greasy silky (milky qz), adamantine (refractive index)
color: a few are diagnostic (azurite, malachite, turquoise), some vary according to
exposure to air (bornite), some by trace composition (Quartz, sapphire, ruby), some by
major composition (pyroxene-talk about effect of amount and color)
streak: color of powder, especially useful for oxides. E.g. hematite always has red
streak, but color not always red.
translucency: metallic oxides often opaque, most silicates, carbonates, sulfates, others
are transparent or translucent if sliced thin enough.
Cleavage, parting, fracture
Planes of weakness in the crystal,
parting is breaking along other planes of weakness, such as twinning surface, exsolution
surface.
fracture-no planes of relative weakness (Qz)
break along planes with weaker bonds, e.g. Van der Waals bonding in graphite, easily
cleaves along that plane.
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Hardness
Mohs scale
Related to bond strength. Different bonds in different directions, so hardness may
depend on direction (kyanite), or which crystal face (calcite)
In general, in increasing bond strength/hardness
Van der Waals, hydrogen bonds, ionic bonds, covalent bonds
Tenacity
brittle, malleable, sectile, ductile
Specific gravity
depends on how closely packed atoms are and atomic mass of atoms. (compare to mass=
like dividing by mass of H2O, makes dimensionless, but because water has a density of
1g/cc, you get the same number)
Magnetism (diamagnetic and paramagnetic and ferromagnetic)
radioactivity, solubility in HCl
Piezoelectric
non-conductor, otherwise it shorts itself out. Must not have center of asymmetry (that is,
its atomic arrangement is different in one direction from the other along some axis, or
polar). Used in altimeters, pressure gauges, timer in watches and computers.
Hydroxyapatite is piezoelectric, important in bone formation.
Reactions, Stability, and Behavior:
Crystallization:
Concept of phases: phases are macroscopically homogeneous regions bounded by
distinct edges.
gases, liquids, solids are the examples of phases you learn in high school. But a
particular material can exist in more than one solid or liquid phase. For example,
graphite and diamond are two solid phases with the same composition (polymorphs).
In gases, individual molecules or atoms have no long range order, and are not bonded to
nearby molecules or atoms.
In liquids, molecules or atoms have no long range order but are bonded to nearby
molecules or atoms but those bonds are not strong enough or persistent enough to
maintain a regular long-range order, although a short-range order often exists.
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In solids, molecules and atoms are bonded to nearby modecules and atoms, most
normally establishing both local and long range order (crystals). Some solid materials do
not have long range order (although short range order typically exists). These amorphous
materials are called glass.
Crystallization occurs when a material goes from a gaseous or liquid state to a solid,
ordered state. This occurs when T, P, composition or other properties change in such a
way that the solid state is energetically favored over the former state.
For example, evaporating water from salt water increases the concentration of Na and Cl
dissolved in the water to the point that salt crystals will form.
Cooling magma will bring the temperature to a value where crystals begin to form in the
melt.
Phase Diagrams, graphical illustration of crystallization reactions and phase transitions.
One-component reactions (different phases of a single chemical component)
Primary variables are T and P.
In general, the phase preferred at higher pressure will be the denser phase.
The phase preferred at higher temperature will be the less well-ordered phase and/or the
phase with higher energy bonding.
Water on blackboard: Phase diagrams illustrate fields of T and P where phases are stable.
Lines represent reactions, such as the reaction in which liquid water freezes to ice (find
that reaction).
Water-salt binary on blackboard, basic ideas of composition change, number of phases,
degrees of freedom,
The six crystal systems in 3-D
(from least to most symmetry)
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show lack of symmetry with parallelogram in 2-D (although point out rotation axis).
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Classification of Minerals:
Crystal structure and symmetry are not the only important characteristics of a mineral.
Chemical composition is also important. Minerals are often classified into mineral
groups.
Mineral groups are based on the primary anion (not cation) of the crystal. This is because
minerals with a common cation usually have more in common in terms of properties than
do minerals with common cations (for example, compare cerrusite and siderite to galena
and pyrite). Also, the anion more consistently reflects the geological environment of
formation. That is, sulfides tend to occur together in one type of environment, whereas
carbonates occur together in a different environment, and silicates in a third environment.
Native elements (metals and nonmetals) (no anion)
e.g. Cu, Au, Fe, Fe-Ni
e.g. S, C
bonds are metallic in metals, or covalent or other in nonmetals
Sulfides (and sulfarsenides, aresenides, antimonides, selenides, and tellurides) (S, As, Sb,
Se and Tl are anions)
e.g. FeS2 (pyrite or marcasite), ZnS (sphalerite or wurtzite), arsenopyrite (FeAsS arsenic substitutes for S), CuS (covellite), Cu2S (Chalcocite), Cu5FeS4 (bornite).
bonds are mainly ionic, although there are also covalent bonds and metallic
bonds.
Usually opaque with distinctive streaks and colors
Oxides (and hydroxides)
e.g. Fe2O3 (hematite), Al2O3 (corundum), Ilmenite (FeTiO3), Magnetite (Fe3O4),
cassiterite (SnO2), goethite (FeO(OH)).
bonds are mostly very strong ionic bonds. These minerals are often very hard. Oxides
are usually very stable minerals.
Oxides are important ore minerals, including Fe, Cr, Mn, U, Sn, Al (although the stability
means that substantial energy investment must be made to separate the metal from the
oxygen). Ruby and Sapphire are members of this group.
Halides
The halides include F, Cl, Br, I, etc. e.g. NaCl (halite), KCl (sylvite), CaF2
(fluorite)
Bonding is the most completely ionic of any of the mineral groups because the
electronegativities of the constituent elements are the most different.
This group has the highest crystal symmetries because ions are spherical and bonds are
symmetrical. Symmetry decreases as cations of higher valence than 1 are involved, and
the bonds become more covalent.
Have the characteristics of ionic solids: e.g. low hardness, poor conductors
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Carbonates: (and nitrates)
e.g. CaCO3 (calcite, aragonite), FeCO3 (siderite), CaMg(CO3)2 (dolomite),
Cu2CO3(OH)2 (malachite), Cu2(CO3)2(OH)2 (Azurite).
Triangular anionic complexes bound more strongly than the complexes are bound
to other ions. Each oxygen has a residual charge of -2/3. Bonding of the CO3 group is
not as strong as CO2 bond, so in presence of H+, the carbonate group becomes unstable,
breaking down to form CO2 and water.
Bonds in the complex are covalent, bonds between complex and metal cations are
ionic.
Calcite structure: Like halite, but with CO3 groups in place of Cl and Ca in place
of Na. Symmetry of the triangular CO3 groups produces a rhombohedral rather than
isometric crystal.
Sulfates:
e.g. BaSO4 (Barite), CaSO4 (Anhydrite), CaSO4۰2H2O (Gypsum).
non-polymerizing complexes.
Nitrates, Borates, chromates, tungstates, molybdates, phosphates, arsenates,
vanadates.
anionic complexes bound more strongly than the complexes are bound to other ions.
Silicates:
Key idea: whether and how the silica tetrahedra are connected to each other by strong
covalent bonding, or whether the corners of tetrahedra connect to octahedral or other sites
occupied by usually-larger cations by ionic bonds.
Nesosilicates: olivine
remind of model that we looked at. Show overhead. Point out that tetrahedra share
corners with octahedra (M1 and M2), not other tetrahedra. no O shared, share in 0
dimensions. review olivine SS series.
Inosilicates: Single Chain Silicates: The pyroxenes.
share in 1 dimension
Draw single chain on board (2 of 4 O shared, 1:3):
Review SS series for The clinopyroxenes (monoclinic) and orthopyroxenes
(orthorhombic):
Draw illustration:
Phyllosilicates (sheet silicates)
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Structure of micas:
Similar to the structure of illite looked at in lab (triangles represent tetrahedra in 2-D,
diamonds represent octahedra in 2-D).
Muscovite KAl2(AlSi3O10)(OH)2 - K between covalent octahedra-tetrahedra
sandwiches, OH substituting for some O in octahedra, Al in octahedra, other Al
substituting for Si in tetrahedra. Dioctahedral because Al is trivalent, not all octahedral
sites are occupied (2 of 3). Is the Si-O ratio correct? Have to compensate for the Al on
the tetrahedral sites. So better to think of the T-O ratio.
Also, consider substituting Mg, Fe for Al (Phlogopite, biotite)
Tectosilicates: share in 3-D (all 4 O shared with another tetrahedron): Draw picture.
Show substitution of Al, charge balance with Na, lead into Albite, then plagioclase SS
series review. Talk about Al-Si substitution and solid solution at length.
Common Rock forming minerals:
Olivine (solid solution)
Pyroxene (solid solution, miscibility gap between ortho and clino)
Amphiboles
Feldspars (plagioclase series and orthoclase)
Feldspathoids (nepheline)
Micas (muscovite, biotite)
Quartz
Clay minerals (smectites, montmorillinites)
Garnet
Kyanite
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Petrology, Lab #1 Rock Scavanger Hunt
Get into teams of 2 or 3 people. Each team will get one point for each rock correctly
collected.
Find each of the following rocks.
1) A rock that tells the story of being deposited on a beach
2) A rock that tells the story of having formed deep under a mountain range
3) A rock that tells the story of having been formed in a swamp
4) A rock that tells the story of having erupted from a volcano.
5) A rock that tells the story of having formed in a salty, arid sea
6) A rock that tells the story of having formed in a fast flowing river
7) A rock that tells the story of having cooled slowly deep in the Earth's crust.
8) A rock that tells the story of cooling slowly in the crust for a while, then erupting.
9) A rock that tells the story of forming under a mountain range, but not as deep as (2)
10) A rock that tells the story of forming in a subducting lithospheric plate
11) A rock that tells the story of forming in a continental rift
12) A rock that tells the story of forming in a volcanic arc at a subduction zone
13) A rock that tells the story of forming near an igneous intrusion
14) A rock that formed in an alluvial fan on the flanks of a mountain range.
15) A rock that tells the story of forming in an oxygen-poor ocean
16) A rock that formed where crystals settled at the bottom of a large magma chamber
17) A rock that formed on a continental shelf, abundant in life but with little sand or mud
from land
After confirming which rocks are "right", return them to their correct locations.
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Petrology, Lab #2 Review of Rock-forming minerals
Have them get into teams of 2 or 3 people. Identify minerals (if more than one mineral,
identify them all!)
After sufficient time (a waited 90 minutes but those who finished well before this were
getting tired by end-took about 25-30 minutes to go through them), go through the
minerals with them, talking about key characteristics. They score their own labs before
handing in.
M11- 3 minerals, pyroxene, phlogopite, Quartz, also is a feldspar or andalucite or
something, not sure.
M16-calcite rhombohedral crystals
M3 fluorite
M7 garnet (andradite) and dolomite (Ca3Fe2Si3O12)
Siderite
Perthite
M9-Biotite and Na-spar
Samples from MN: hematite, and jaspar
pegmatite: tourmaline, Na-spar, quartz, k-spar
granite with polished side: K-spar, Na-spar, qz, biotite, hornblende (what say about P?
Crystallization sequence?)
158 fluorite with calcite
13 hornblende syenite (K-spar, perthite or Na-spar)
87 garnet (Fe3Al2Si3O12) (pyrope ss) and silliminite
215- amethyst (how distinguish from fluorite?)
35A dolomite (how distinguish from calcite?)
46A Chalcedony
10ab pyrite (with striations and cubes)
46E1436 Calcite cleavage rhomb
m1 alabaster gypsum
m2 selenite gypsum
46E4887 specular hematite
m13 pyrite
sphalerite
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Petrology
Sedimentary Petrology
Terms related to deposition
Detrital = transported fragments and particles
Clastic = fragments and particles that may not have been transported.
Chemical and biochemical = precipitated from water
Major rock types
Sandstone (20-25%): clastic rock with particles 0.06 to 2mm.
Mudstone (65% of sedimentary): clastic rock with particles less than 0.062mm
Carbonate (10-15%): usually chemical or biochemical rock made of carbonate minerals,
particularly calcite, aragonite, dolomite and some siderite and magnesite.
Evaporites: Chemical rock, usually formed from evaporation of sea water, or terrestrial
alkaline or salty waters, in arid, restricted basins.
First three make up >95% of sedimentary rocks.
Problems in classification:
A carbonate might be made of clastic fragments (either transported or not), such as large
fossil fragments in a limestone, or wave-worked shell fragments in a coquina.
Variable amounts of clastic clay can mix with carbonate (Marl).
Age distribution of sedimentary rocks
Half of sedimentary rocks are 130myo or younger (Cretaceous or younger).
Exponential decline in exposure as go to progressively older rocks.
Does that mean that sedimentary rocks form more commonly today than in the past?
No, is related to a roughly constant probability of destruction by erosion, with a certain
fraction of older rocks surviving to later time periods.
Common depositional settings
Most sedimentary rocks are deposited in marine (as opposed to terrestrial) environments.
This is because oceans constitute a larger fraction of earth’s surface, because marine
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environments are more likely to be depositional rather than erosional, and because the
sediments are more likely to be covered by later sediments rather than eroded.
Due to fluctuations in sea level, shallow marine water invading continental areas has been
more pervasive at various times in the past than they are today. These shallow seas are
called epicontinental seas.
Depositional Basins (regions either significantly below base level, or where persistent
subsidence provides room for deposition over extended time periods.)
activity: in groups, try to identify key plate-tectonic environments, and the general
characteristics of sediments deposited in each.
Oceanic Basins: deposits underlain by oceanic crust (basaltic rather than granitic)
a few key considerations: water depth affects light penetration, fossil materials are
often pelagic. Deeper, colder water is more acidic, there is a depth below which
carbonates are not stable and an even greater depth below which settling
carbonates do not accumulate.
Arc-trench system basins: complex system of basins related plate covergence and
subduction.
a few key considerations: extensive tectonism and metamorphism makes these
regions complex. Often associated with volcanic input. Basins range from
extremely deep to not so deep, and may have either oceanic or continental
material base. Sediments include mélanges and turbidites, to more fluvial,
deltaic, marine as get closer to the continent.
Continental collision basins: basins that develop where continents converge
a few key considerations: include elements of ocean basins prior to convergence,
such as ophiolite, and deposits related to the orogeny such as flysch
(synorogenic clastic wedge with marine) and molasse (synorogenic clastic
wedge-terrestrial-e.g west of appalachians or east of rockies) deposits.
Basins in displaced terrains (exotic or “suspect” terrain):
Key considerations: are tacked onto the edge of a continent by plate movements and
so have structural, stratigraphic, and paleontological discordances with the rest
of the continent.
Divergent Grabbens: basins that develop during continental divergence
Key considerations: are often on presently-stable continental margins where
past divergence of oceanic-basin-formation occurred. volcanics, intrusives
common, interbedded with arkosic red beds. In arid climates, evaporates occur.
Intracratonic basins (regions of subsidence in the interior of stable continents)
Key consideration: Deposited in epicontinental seas (non-orogenic, shallow
water), with very thick sediments grading laterally into much thinner sediments
of similar type. (e.g. Williston Basin, Michigan Basin).
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Soils
Horizons:
A (zone of leaching (more extensive leaching in humid), organic accumulation
B (Zone of accumulation (more soluble accumulates in drier)
C (bedrock is altered
Bedrock
Soil formation is a combination of chemical processes weathering rock and minerals and
activity of living organisms (technically, only called soil if it is affected by living things,
thus the lunar "soil" is more appropriately called a regolith).
Key factors:
Water, and leaching vs evaporation
acidity (pH)
oxygen (Eh)
chelating agents (organic molecules that latch onto metal cations and make them soluble)
Solubility in water: ionic compounds more soluble e.g. Na and Cl, Ca and Fl, K, Mn2+,
Fe2+. B3+, P5+, S6+, and C4+ are soluble as oxide anions (e.g. CO32-, PO43-). Covalent
bonded compounds are less soluble (e.g. Fe3+, Al3+, Mn4+, Si4+, Mg2+), often
insoluble as oxides or hydroxides.
Acidity affected by several factors, including:
abundance of CO2 from plant and animal life, producing carbonic acid,
organic acides
Roots exchange H+ ions for nutrient cations in
soil (method of uptake), increasing soil acidity
All of these involve life, so more life, higher
acidity is very typical
Solubility influenced by acidity (see diagram):
3 Kspar + 2H+ + 9H2O = Kaolinite + 4
H4SiO4+2K+ (kaolinite = Al2Si2O5(OH)4
Kaolinite + 5H2O = 2Al(OH)3 (gibbsite) + 2
H4SiO4
plagioclase + H+ + H2O = montmorillinite
(Na and Ca) + silicic acid + (Na+, Ca++)
ferromagnesian + H+ + H2O =
montmorillinite (Na, Ca, Mg) + muscovite+
kaolinite + gibbsite +silicic acid + ferric
hydroxide (which forms hematite or geothite).
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Eh: affects valence state, thus solubility (e.g. Fe3+ insoluble, Fe2+ soluble; Mn2+
soluble, Mn4+ insoluble)
Chelating agents:
otherwise insoluble elements (e.g. Fe3+, Si4+) attach to chelates, and then become
soluble. Increasing Eh can destroy chelating agents, causing an effect of Eh other than
valence changes.
Petrological significance:
There are multiple soil classification schemes, with perhaps 16800 soil species, not
always classified exactly the same (USDA classification not same as international
although significant similarities). Can't identify all these in rock anyway.
Useful to think of two types of soils, pedalfers (rich in Al, Si), pedocals (rich in Ca).
A horizons often not preserved.
General ideas:
Fe or Al rich soils (laterite, bauxite) indicative of extreme leaching, typical of humid
tropical. Aluminocrete and Ferricrete are soil horizons often preserved geologically.
Silicrete also often preserved, where Si is associated with laterite, suggesting the mobile
Si was removed in some areas and deposited in others.
Kaolinite rich indicates humid tropical also. maturity of minerals indicate degree of
weathering and maturity of soil. Thickness of leached zone, and severity of leaching
gives clues to rainfall.
CaCO3 (as in calcrete, another often-preserved soil remnant) indicates arid climate.
Depth of the calcic nodule horizon, (or whether it is continuous or nodular) gives clues to
paleorainfall.
Sandstones and Conglomerates
Studied a lot because particles are big enough to see and study.
classified by particle size and type (composition) of particle
Particle Size (phi scale = -log base 2 of particle size in mm-so 1mm = 0, 1/4mm = 2,
2mm = -1 etc)):
Gravel = 2mm to 4096 mm (-1 phi to –12 phi) (granule, pebble, cobble, boulder)
Sand = 0.062mm to 2 mm (4 phi to –1 phi) (fine, medium, coarse)
Silt = 0.004mm to 0.062mm (8 phi to 4 phi) (fine, medium, coarse)
Clay = <0.004mm (greater than 8phi)
Classification by particle type (of sandstones):
draw on board
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Interpreting Sedimentary Rocks: 1) Present is the key to the past (go look at modern
environments and carefully document the sediment details). 2) consider what processes
occur in different environments and how these must necessarily affect the sediments. 3)
consider the setting of the rock (including the pattern of co-occuring features and the
stratigraphic position).
Textural Maturity: (rounding, sorting, size, mineralogy) reflects distance and energy of
transport, but also degree of working (kinetic energy) in the environment of deposition.
Thus, river sediments tend to be much less mature than beach sediments where waves are
constantly working the grains.
Rounding: Factors to consider include resistance to mechanical (or chemical) weathering
which preferentially takes off exposed corners, but also cleavage which tends to limit
rounding. So, mica will never round, feldspar not too readily, but qz, with no cleavage,
will round well, albeit slowly because of chemical and mechanical resistance to
weathering.
Very high degree of rounding in quartz grains is associated with wind-blown
sand, and, to a lesser, degree, with beaches.
Sorting of grains: Small clay and silt is quickly removed from a sediment at its source,
larger particles are sorted according to size by the transporting medium. Sorting reveals
something about transport distance, but usually more about environment of deposition.
e.g. Beach environments are particularly effective at sorting sand.
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Illustration of skew and tails, and bimodal distribution
The sorting and distribution of sediments reveals various processes that have acted on a
sediment.
A skew with a tail toward coarse sediments (negative phi), might reveal that the sediment
was winnowed by a medium with insufficient energy to remove larger particles. For
example, wind can move smaller particles very efficiently, but not larger particles.
Sometimes beach sand may have such a skew if it is built on a base that includes
coarser sediment that the waves lack the energy to remove.
A skew with a tail toward fine sediments (positive phi), might occur in stream deposits
where the coarser sediments are deposited when the river is higher, then, as it wanes,
finer sediments filter into the sand, creating a fine “tail”.
A bimodal distribution often indicates two separate processes, such as fine mud deposited
from water in a lagoon, with coarser silt or fine sand blown in by wind. Or,
bioturbation may mix two vertically adjacent sediments that were deposited in
different environments. Bimodality might also reflect the nature of the provenance
material.
Grain type:
igneous and metamorphic rocks are about 20% Qz, very little clay minerals.
Sedimentary rocks are 45% clay, 40% Qz, 6% feldspar, and 5% disaggregated
rock fragments, 4% the rest.
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More chemically and mechanically stable minerals survive, and clay minerals are
formed by weathering processes (unstable minerals include olivine, anorthite; more
stable include quartz and orthoclase, accessory minerals like magnetite and zircon often
stable in sedimentary environments)
So grain type can indicate transport duration. However, grain type can carry other
information as well.
For example, Feldspars are mechanically resistant but chemically unstable. Therefore,
they can be indicators of climate. They weather much less quickly (more like quartz)
in a very cold climate, and much more quickly in a wet, tropical environment.
Garnet, kyanite, muscovite are metamorphic minerals, and can indicate a metamorphic
Provenance.
Zoned Plagioclase is typical of igneous rocks (explain relative to phase diagram we
studied before, what causes zoning: changes in melt composition, crystal
composition, and reequilibration rate controlled by diffusion).
Sanidine is a volcanic rock (high T igneous form of K-spar that is less common in
granite)
Ilmenite, chromite, augite, and plagioclase are typical of intrusive igneous rocks.
Euhedral hexagonal biotite flakes indicate air deposition from volcanic ash (they would
have lost euhedral shape if weathered or abraded).
Glauconite, usually indicates formation in a marine environment from fecal pellets
Lithic fragments indicate rapid deposition before significant weathering occurs and can
yield significant information about the provenance, depending on the type of
fragment (igneous, metamorphic, sedimentary).
Quartz or other grains may retain information about what type of rock they are from, such
as presence of quartz overgrowths from a previous episode of sandstone
cementation, 120 degree interfaces typical of metamorphic rocks, undulatory
extinctions due to strain in crystals (less typical in volcanic quartz than those
subjected to deformation), milky quartz is typical of hydrothermal quartz (due to
presence of tiny cavities filled with water).
presence of heavy minerals, or their size distribution, can indicate depositional
environment. e.g. wind blown sand often has higher heavy fraction (magnetite) than
beach sands.
Example studies (from Colson et al. 2004):
effect of transport medium on size of equivalent particles. Think about settling rate (not
completely the full problem, but helps to understand concept). Think of a block of wood,
a grain of quartz sand, and a grain of gold. What will be their relative fall rates in a
vacuum? In air? (gold fastest, wood slowest), in water (wood never settles, bigger
difference in settling rates of other particles).
Based on the size of hydraulic equivalents, can sometimes infer depositional medium.
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mean mt size (mm)
0.13
0.12
wl1
small x-set
0.11
wl2
0.1
transition
0.09
marmarth at lmr
fox hill at lmr
0.08
marmarth
0.07
0.18
0.23
0.28
mean grain size (mm)
measured
pred. air
pred. water
Fig. 7. Measurements of magnetite/sediment average sizes compared to the ratio of sizes
expected for transport in either air or water medium. Average particle sizes were determined by
measuring sizes of randomly selected particles under a microscope. Sizes for magnetite
particles were determined by measuring particles separated magnetically from the sample and
identified as magnetite under a microscope and measured.
Hydraulic equivalence is defined as follows: let HN = (DV-BV)/(AF)
where HN=hydraulic number, D=density of particle, V = volume of particle, B=buoyancy force
of medium per unit volume, A=cross-sectional area of particle, and F=force of the transporting
medium exerted per unit area. Then hydraulic equivalence is achieved if HN(magnetite) =
HN(sediment). Sediment density is assumed to be approximately that of quartz. Lines on the
graph illustrating expected (pred. = predicted) relationships for hydraulic equivalence in either
air or water are calculated from this relationship.
Sample locations:
Foxhills at lmr: upper Fox Hills Sandstone from the Little Missouri River (Frye, 1969, section 9, Bowman Co ND) (LM12).
Small x-set: Unit 2 at east end of CHS (708-SB-2).
Transition: Unit 3 at West Ledge of CHS (707-WL-3).
Wl1: Rooted Sandstone west ledge (708-WL-1).
Wl2: Rooted Sandstone west ledge (708-WL-2), wl1 and wl2 are from 2 different cross sets of Unit 4 and are 30 cm apart vertically.
Marmarth: lowest Marmarth sandstone from about 4 miles south of the bone bed (SS4).
Marmarth at lmr: lowest Marmarth Sandstone from the Little Missouri River (Frye (1969) section 8, Slope County ND) (LM5).
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From the same study. Unit 3 interpreted as shoreline deposit, water worked but with
wind-deposited material also. Unit 4 wind deposited. Unit 6 river deposited or deltaic.
Note the fine tail in shown for Unit 6.
Table 2. Sorting and Rounding
Sorting
Rounding
(average)
Unit 3
0.85
2.2
0.6
2.6
1.2
1.6
707-WL-3
Unit 4
708-WL-1 and 2
Unit 6
712-BB2-9
Simple sorting measure (Inman, 1952),
5=rounded, 1=angular.
Determined by observation and measurement
of loose grains under a microscope
use overhead for bar graph.
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Sediment analyses from the science center cutbank (student research). Note the good
sorting of inferred beach sand, the tail on several water-washed glacial drift, the poor
sorting of till, the good sorting and fine grain size of wind-blown sediment. (use
overhead)
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Mudstones
Due to small size (clay minerals often can’t be studied effectively even under
microscope), it is often difficult to extract information. The particles are too small to be
meaningfully separated by a sieve.
Grain size.
The proportions of clay and silt can be used to infer environments. For example, deltaic
sediments are typically silty where some stream flow or water movement occurs, whereas
lagoonal sediments may be more clay rich. The proportions of silt and clay can be
measured by disaggregating the sample (which may not be easy), and measuring the
settling rate of sediments in a column (an hydrometer analysis).
problems:
1) you may not completely disaggregate the rock
2) The particles may have originally settled as flocculants, or been deposited as fecal
pellets, thus the disaggregation may not give you a measure of the actual hydraulic
particles meaningful in the depositional environment.
Color:
Black is indicative of either free organic carbon, or magnetite or sulfides. Free organic
carbon indicates higher organic production than decomposition, usually indicating a
reduced environment.
Red and brown indicate oxidizing conditions, typically terrestrial (hematite and goethite)
(FeO(OH)).
Green color occurs in the absence of ferric oxides and hydroxides due to illite, chlorite,
and biotite in typical shales (indicating more reducing environments where Fe is soluble
and doesn’t produce redish colors).
Other textures:
laminations, indicate depositional processes
Soft sediment deformation features (water escape, or overloading features)
Bioturbation and pelletization
preferred orientations of platy or linear grains (preferential axis of elongation of
paramagnetic minerals can allow orientation to be measured magnetically rather
than microscopically)
fissility (depends on orientation of platy minerals and organics, decreases with
increasing flocculation which is maximum in about 2000ppm salinity-sea water
~30000ppm by weight)
dessication cracks.
Example: Pierre Shale study
Bentonite (colloidal silica and smectite, an expandable clay (Van der Waals and
hydrogen bonds between layers of silica tetrahedrons-weak bond allows H2O into the
20
layer). Indicates volcanic contribution. Also Crystobalite (determined by X-ray
diffraction), a higher T, low P version of SiO2, indicative of volcanic origin.
Source direction identified. And further evidence of volcanic input (increase in
Feldspar)
Limestones and Dolostones:
Grains (allochems)
fossils: pelecypods, brachiopods, ostracods, crinoid, bryozoans, etc Shape identifies
in hand sample, internal structures and characteristic shapes identify under
microscope
ooids: sand size concentric, spherical polycrystalline carbonate grains. Form by
repeated layering of a nucleus (e.g. qz, fossil), need to be agitated wo typically
indicate shallow environment, wave-swept shoal, often cross bedded, other
sandstone features.
peloids: spherical or ellipsoid carbonate mud pellet with no internal structure. Often
fecal pellets (giving uniform size). Certain boring algae can destroy internal
structure of other types of particles, creating a peloid.
Limeclasts: fragment of lithified or partially lithified preexisting limestone.
Intraclasts, is a subset of limeclasts that originate penecontemporaneously from
within depositional basin, transported e.g. by storm.
21
Matrix (orthochem)
Micrite: clay-silt sized particulate calcium carbonate deposited with grains. Origin
often is algae, or chemical precipitation from sea water. Usually deposited as
aragonite that later converts to calcite.
Spar: Crystalline calcium carbonate grown in voids in rock, or recrystallized micrite.
Origin is often inorganically precipitated CaCO3 during
Classification
Folk: first name based on allochems, primary name based on matrix (orthochem).
E.g. oosparite, pelmicrite. Add additional modifiers: trilobite intrasparite, crinoid
biomicrite, etc.
Dunham: first name again based on allochems, primary name based on whether the
rock, absent the matrix, is grain-supporting.
Mud supported= mudstone (less than 10% grains
= wackestone (more than 10% grains)
Grain-supported = packstone (contains mud)
= grainstone (no mud)
original components bound together during deposition (such as scleractinian corals)
=boundstone
Dolomite formation has long and controversial history. There is no solid solution
between calcite (or aragonite) and dolomite, although Mg solubility is a function
of T. Dolomite and limestone do not occur interspersed with each other.
Dolomite is associated with limestone and evaporites. Probably more than one
mode of formation, postdating deposition.
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Petrology, Lab #3
Sedimentary Rocks and their Interpretation
sample numbers such as 26A refer to samples collected at MSUM pre-Colson and are in
rock cabinet 1. samples such as 47E4674E are samples from Wards, some of which have
thin sections with them. They are also in cabinet 1. Samples such as W48 are part of the
Wards rock sample set and are in rock cabinet 2. IRS refers to introductory rock samples,
and are kept either in the metal rock cabinet, or nearby it. I list 10 samples to look at
more carefully, and a number of samples to also examine and think about (learning to
identify rocks entails seeing a lot of varieties of the same kind). I suggest that the keep a
rock-log, writing down key characteristics of rocks and the stories that the rocks tell. On
the Final Exam, I will ask you to both identify rocks and tell their story. DON’T PUT
ACID ON THE SAMPLES FROM WARDS CABINET SETS, OR BASH THEM!
Graywacke and Arkose: Both of these rocks are composed of immature sediments,
angular, poorly sorted, with unstable minerals and/or rock fragments. They are both
deposited near their source after relative short transport and rapid deposition, and are both
often associated with orogenic events..
Arkose: Often derived from a granitic source region, contains muscovite, orthoclase,
quartz. Commonly flank mountains where alluvial fans form where streams emerging
from the mountains lose energy. Typical red color from Fe-oxide cements (common in
terrestrial environments)
samples: the IRS arkose sample in the introductory lab case is from the Fountain
Formation. It was deposited on the flanks of the Ancestral Rockies, a range of mountains
that existed in mid Colorado in the paleozoic. This is a classic arkose.
Note the features consistent with the interpreted environment of deposition and source
region.
also look at the Triassic Sugarloaf Arkose from Massachusetts (W50).
Graywacke (26A): Often derived from a volcanic source region. They are often
associated with flysch deposits or turbidites in association with convergent plate margins.
Note the features consistent with this interpreted environment of deposition and source
region. Lack the reddish color typical of terrestrial arkose sediments.
Note the features consistent with its interpreted deposition and source.
23
Also look at the Upper Devonian Graywacke from New York (Acadian Orogeny) W51.
Sandstones:
Sandstones form in environments where energy is appropriate for deposition of
sand sized particles. This can include deposition by wind or water, and deposition in
terrestrial or marine settings. A few observations are these: beach and wind-blown sands
are usually quite mature (mostly quartz, well-rounded, well sorted). Red coloration from
hematite cement often occurs in terrestrial deposits. River sands are usually less mature,
with more angular grains, are more poorly sorted due to higher silt fraction (“dirty” sand),
may contain more unstable grains like orthoclase, micas, etc. Green glauconite usually
indicates a marine source since glauconite often forms from fecal pellets.
Examine the following samples:
37B: Navajo Sandstone. Terrestrial, wind-deposited sandstone.
cite features consistent with this.
46B: Sandstone, Burning Coal Vein ND. Terrestrial, river
cite features consistent with this.
W48: Glauconitic sandstone. Interpretation?
Also look at the sandstones: W45, W46, W47 (argillaceous refers to clay content), and
the sandstone concretion (highly cemented region of sandstone) 24B, 27b, 47E7064
(fossiliferous). Look at the IRS sample from the Wisconsin Dells. It is a supermature
beach deposit. Notice and cite features consistent with this interpretation.
Conglomerates represent deposition in a higher-energy environment than sandstones.
Also look at the conglomerates: 29A, 26B, 40B (notice the scoria and obsidian rock
fragments!- hematite caliche cement suggest desert-soil related formation), W44.
47E2224.
Shales represent deposition in a lower-energy environment than sandstones. Also look at
2B (green river shale is terrestrial intermontane lakes and streams), 31b, IRS claystone
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from the Golden Valley Fm (terrestrial river-lagoon), 38B (El Capitan, marine), W49
(siltstone), W52, W53, W54, 41A, 47E7084, 47E7404, 47E7414, 47E7474.
Limestones: Limestones are mostly biochemical, and mostly marine. They represent a
variety of environments, including wave-swept beaches or shoals (e.g. oolitic or coquina)
various marine shelf or reef (e.g. fossiliferous limestone), and other environments.
Look at the following samples, and interpret their environment of formation
W59 crinoid sparite or crinoid packstone (compare with IRSR4 crinoid micrite or crinoid
wackestone)
34A (coquina or grainstone)
47E4674 (oosparite or oolitic grainstone)
IRS R6?
! Think about this one. I find it very intriguing. Notice the rock fragments (noncarbonate-try acid on them!), the shells, and the micrite mud.
W60 (Fremont Limestone): The red color is from terrestrial mud that washed down into
caves formed in the Paleozoic Fremont during the Mesozoic period. This type of
mottling from insipient cave formation is not atypical of limestones.
Also look at W63 and 47E4664 (chalk), 47E4609, 90, 86, 25B, W62, W64
Other chemical and biochemical sedimentary rocks. Also look at 35a (Dolomite),
47E4694 and W65 (dolomitic limestone), 38A (halite) 15A and W69 (rock gypsum),
28A, W56, and W61 (chert), 21A (flint), 50A (peat), W55 (Bauxite, formed by
weathering), W58(diatomaceous earth)
25
Petrology, Lab #4
Sedimentary Rocks in Thin Sections:
Using a petrographic microscope entails not only looking at the sample in polarized light
(called ‘Plane’ light), but looking at the sample under “crossed polars” or “crossed
Nichols”. Polarized light is light which is vibrating in only a single direction (polarizing
sunglasses only allow light through that is vibrating in a particular direction). The light
source for a petrographic microscope is polarized. This “plane” light goes through the
crystalline samples, which can split the plane light into new light components that may
not be polarized in a single plane any more. “Crossing the Nichols” involves inserting a
lens between your eye and the sample which only lets light pass when it is vibrating in a
direction perpendicular to the original plane light (this means that if you look into the
scope with no sample and with the polarizers crossed, you will see only blackness since
none of the plane-polarized light is allowed to pass the “analyzer” lens, which is what the
second polarizing lens is called. Because the way that the crystal changes the light
depends on the exact details of the crystal structure, the light-polarizing and analyzing
capabilities of the petrographic microscope give one all kinds of ways to learn about
crystals.
Thin sections for this lab are kept separately with the microscopes and have various
sample labels. PLEASE TAKE CARE OF THE MICROSCOPES AND THE
SLIDES!!!!
Arkose (with hand sample):
Thin section 8. Look at the sample using the lowest power lens. First look at the
sample under plane light (not plain light!). The analyzer lens, which is on the right hand
side of the optic column between the sample and the eye lens, should be out. The sample
should look mostly clear with reddish brown stringers. You can see distinct mineral
grains that make up the rock.
The reddish brown streamers comprise the iron-oxide cement between mineral grains.
Find it.
There are a few small black minerals. These are opaque (light is not transmitted through
them. Find them. Common opaque minerals include oxides and sulfides. Based on the
environment of formation of this rock, do you think these are likely to be oxides or
sulfides? Why?
There are some brownish or greenish mineral grains that are not part of the cement. Find
some of them. These minerals are pleochroic. This means that when you rotate the stage
in plane light they get lighter and darker (either brown or green). This is a characteristic
feature of two micas, either biotite (brown) or chlorite (green). Notice how the biotite
and chlorite are mixed in with each other (as the biotite is reacting to form chlorite).
Notice the poor sorting and poor rounding of the grains. What does this tell you about
the formation of this rock?
26
Now, cross the Nichols by inserting the analyzer lens (push in the lens on the right hand
side of the optic column). YOU NEVER HAVE TO ‘FORCE’ ANYTHING ON THE
MICROSCOPE....TREAT IT VERY GENTLY!!!!
Rotate the stage a little bit and notice that the grains go alternately dark and light. This
has to do with the way that the plane light is being changed by the mineral, and then
“analyzed” by the crossed polarizing lenses. When it gets dark, we say that it has gone to
extinction. This occurs when crystallographic axes in the mineral exactly line up with the
polarizing and analyzing lenses. This allows you to see not just the shape of particles, but
the actual individual crystals in each particle. If you look around, you may be able to
find some particles that are made up of many small crystals. These are rock fragments
(which tells us about the immaturity of the sediment in this rock).
Notice also the new colors of the minerals. The colors you see when the analyzer lens is
in are called interference colors. Interference colors are a result of diffraction that occurs
due to the way the crystal interacts with the light and the polarizing lenses. Interference
colors are characteristic of each mineral. Most of the minerals you see in this sample will
be grey or light yellow. These are quartz and feldspars. You can tell the feldspar from
the quartz because the feldspar will be twinned. Find the black and white straight
patterns that mark the synthetic twinning typical of feldspars. There is also some
carlsbad twins around, which you might spot as being a twin that is on a bigger scale than
the small “railroad track” synthetic twins.
Now, with the Nichols crossed, look around for some really bright colored minerals
(bright orange or blue or red). Uncross the polars briefly to see if the mineral looks clear
in plane light. This is muscovite, another mineral typical of Arkose.
Sandstone (with hand sample)
Thin section 7: Begin looking at this sample in Plane light. Notice the much
better rounded grains than in the arkose sample. The reddish brown goo between the
grains is hematite cement. (What do these two observations tell you about the formation
of this rock?)
Cross the polars and look for feldspar and muscovite (notice that there is not
much, if any- what does this tell you about the formation of this rock?)
Cross and uncross the polars a few times, also rotating the stage, and notice that the shape
of the rounded grains does not match the shape and size of the actual crystals (each
separate crystal being defined by the extinctions of the individual crystals). The “extra
crystal” beyond the edges of the apparent grains are quartz overgrowths that formed on
the original quartz sand grains. Thus, the quartz overgrowths have filled in most of the
original void space in the rock, forming a very strong quartz cement.
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There are some void spaces remaining. When you cross the polars, the void space is
always black, no matter how you rotate the stage. Find some of these void spaces (now
filled with epoxy of course).
There is a second type of “always black” region that is not a void space. If a quartz
crystal happens to be oriented so that we look exactly down the “c” axis, it does not
change the polarized light, and the result is that the polarized light maintains its same
orientation through the quartz crystal and then is “cut off” by the analyzer lens which
only lets light through that is perpendicular to the original polarized light direction. Find
a quartz crystal that you are looking down the “c” axis at.
Center the “c axis” quartz crystal on the stage, then rotate in the high power lens and
refocus (BE VERY CAREFUL, IT IS EASY TO RUIN THE MICROSCOPE OR THE
SLIDE BY RAMMING THE HIGH POWER LENSE INTO THE SAMPLE AS YOU
FOCUS!!!). Now, engage a new lens, called the Bertrand lens. This lens is on the left
hand side of the optic column. You should see a small cross when you look into the
scope. This is called an optic axis figure and can allow one to make important inference
about the crystal structure of a mineral. Find a different quartz crystal, one that gets very
bright as you rotate the stage, and do the same thing. The cross is still there but it is off
center. As you rotate the stage you can watch the different legs of the cross as they cross
the field of view.
Limestone (sample D-26) (biowackestone, or biopelmicrite)
Things to find:
Orthochems:
micrite and spar (the spar will have bigger crystals of calcite, which will look clear in
plane light with high order interference colors (bright reds, blues) with crossed polars).
Micrite looks “muddy” and you can’t see individual crystals even under the microscope.
Allochems:
This sample has a variety of fossils as well as peloids. Peloids are more abundant in the
top half, fossils in the bottom half.
Trilobites: characteristics include a “shepherd hook” shape, and an extinction that
sweeps from the center outwards toward each end. Due to recrystallization, the wavelike extinction is only sometimes seen in this sample.
Brachiopods: Characteristics include two layers to the shell (not visible much in this
sample due to significant recrystallization of this early paleozoic sample), and a fabric
that cuts at a shallow angle relative to the trend of the shell.
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Bryozoa: Characteristics include a cellular pattern formed by the openings where each
zooid lived (this is distinct from the cellular pattern of coral.....I didn’t find any coral in
this sample).
Mollusc fragments: (characteristics include the shell being replaced by calcite crystals,
producing a mosaic appearance to the crystals (visible under crossed Nichols as you
rotate the stage).
Peloids: Round structureless blobs.
Other things to notice:
Several brownish stringers run through the sample. These are mini-stylolites, dissolution
features that form when minerals dissolve when the rock is under pressure.
Some conversion to dolomite has occurred. This can best be seen in the red-stained area.
The calcite stains red, but the dolomite does not. Notice the rhomb-shape of the dolomite
crystals.
Limestone (sample 44-7362) (with hand sample)
Notice the concentric nature of the oolites. These form where waves continually lift and
redeposit them, such as a shoal.
You may find pieces of brachipod, echinoderm, or foraminifera at the cores of some of
the oolites and coated fragments.
Sandy sediments and environment of deposition
Examine each of the following sandy sediments under a binocular scope (reflected light)
and cite evidence for each depositional environment:
Padre Island (wind blown dune sand derived from beach)
Wisconsin Dells (supermature each sand)
Elk Lake Montana (immature sand derived from glacial till)
29
Petrology. Lab 5: Igneous Rocks in hand sample, Textures and
Compositions
Examine the following hand samples. For each sample, use the rock name, along with
information in your book and examination under a hand lens to try to identify major
minerals in it (1-4 as indicated). Secondly, list the appropriate key minerals [those from
the classification diagram: quartz, plagioclase, alkali feldspar, feldspathoid] with their
approximate proportions. Finally, for each sample, identify its key texture and give the
most likely interpretation of how that texture occurred. Key textures include such things
as aphanitic, phaneritic, porphyritic, vitrophyric, ophitic, poikilitic, vesicular,
amygdaloidal, veining, etc.
Calc-Alkaline family
Wards 2, muscovite, biotite granite
Identify minerals (4):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 12, rhyolite porphyry
Identify minerals (2):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 27 Hornblende Andesite
Identify minerals (1):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 7, Granodiorite
Identify minerals (4):
(be sure to notice the difference in proportion of mafic minerals when compared to granite)
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Others to identify: rhyolite, rhyolite tuff, syenite
30
Mafic/ultramafic family
Wards 25, Diorite
Identify minerals (3):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 30, Olivine gabbro
(this sample is missing, if you can’t find it, use the sample of gabbro in the metal
cabinet with the introductory physical geology samples.)
Identify minerals (2):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 32, Anorthosite
Identify minerals (1):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 35, amygdaloidal basalt
Identify minerals (1):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 43 serpentinite
Identify minerals (0):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Others to identify: diorite, diorite porphyry, diabase, diabase porphyry
31
Alkaline family
Wards 13, hornblende syenite
Identify minerals (3):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 18, Nepheline, sodalite, syenite
Identify minerals (4) Note: sodalite fluoresces under black light:
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 42, kimberlite, a mafic alkaline rock, (be sure to notice the xenoliths!!!)
Identify minerals (0):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Wards 23 latite porphyry
Identify minerals (3):
Key minerals from classification diagram, with approximate proportions:
Identify and interpret texture:
Others to identify: olivine lamproite
Attention: don't read below until you have looked at samples and tried to identify minerals!
Minerals visible in hand sample:
25: hornblende, plag, sphene, biotite
30: plagioclase, olivine
27: hornblende
32: anorthositic plagioclase
35: Quartz, calcite, epidote/chlorite in amygdules
43: magnesite, pyrite, serpentine
13: alkali feldspar, hornblende, biotite
18: alkali feldspar (with twinning), nepheline, sodalite, amphibole
42: serpentinized olivine (porphyritic), large xenoliths.
23: plagioclase, biotite, pyroxene
32
Lab 6: Igneous Rocks in Thin Section
Thin Section 3, Gabbro
Find the Olivine: it has an erratic fracture pattern (not the regular linear fractures
indicative of good cleavage). It is a faint greenish color in plane light. Under crossed
Nichols it has high interference colors, red/blue/green.
Find Plagioclase: it is lath shaped, or rectangular. It has low interference colors under
crossed Nichols (grey/white). It exhibits very distinctive polysynthetic twinning, visible
under crossed Nichols by the alternating bands of extinction as the sample is rotated.
Find Biotite: It is brown or green and pleochroic (as in the sedimentary samples). It has
a birds-eye texture in plain light (resembles birds-eye maple).
Notice the opaque minerals.
Find the pyroxene: it is a light brown color in plane light. It has medium
interference colors under crossed Nichols (orangish). fractures run parallel to
each other, indicating cleavage. In many grains, two sets of fractures intersect
at nearly right angles (2 cleavage planes at right angles to each other).
There is some of another type of feldspar, generally without polysynthetic twinning,
which contains poikilitically enclosed olivine crystals.
Notice the serpentinized veins (small alteration veins): they are greenish in plane light.
where the veins intersect opaque minerals or olivine, biotite often occurs. Opaques and
olivine are also altered to biotite along the veins.
Thin Section 1, Granite
Find the quartz: equant crystals with low interference colors (grey to yellow). No
cleavage planes, usually don’t show an extensive fracture pattern either.
Find the feldspar: there are two kinds. Na-rich plagioclase shows polysynthetic
twinning. K-spar grains are quite large and have a mottled extinction appearance under
crossed Nichols.
Thin Section 2, Diorite
find the amphibole (hornblende): light brown to dark brown or green,
pleochroic, cleavage fractures run at angles of about 124 degrees. Medium
(orangish) interference colors.
There is a particularly large hornblende grain. Find it, and notice that it is
ZONED (meaning that its compositions changes outward from the middle of the crystal).
33
This is due to changes in the melt during crystallization (think about phase diagrams!).
Notice also that there are abundant poikilitic inclusions in the greener-colored rim. The
rim is probably richer in Na and Fe than the interior.
There is some biotite. You can distinguish it from hornblende because it will generally
have different pleochroic colors slightly, only one dominant cleavage apparent (and
pleochroic colors will be darkest when the cleavage plane of the sample is aligned with
the polarizing lens), birds eye appearance, and higher interference colors than hornblende
(red/blue). Also, the grain will go to extinction when the cleavage planes are parallel to
the polarizing lens (parallel to the cross hairs), whereas amphibole goes extinct when
there is a distinct angle between the cleavage planes and the polarized light orientation
(parallel to the cross hairs).
find the polysynthetically-twinned Plagioclase
Notice that quite a few of the opaques (ilmenite and magnetite) are diamond shaped.
Based on their crystal systems -you should still have your mineralogy book- are the
diamond shaped opaques more likely to be ilmenite or magnetite?
in Plane light, find some the hexagonal poikilitic crystals in hornblende. These are
apatite (check out the crystal system for apatite in your mineralogy book). Notice that
many of the most perfect hexagons are always black under crossed Nichols. This occurs
when you look right down the c axis of the crystal because the speed of light, while
different along the c axis, is the same in every other direction in the crystal.
I also found a grain of titanite (diamond shape, high interference colors), light brownish
color.
Wards 44E6119 Diorite Porphyry (compare this to Thin Section 2)
Find the euhedral to subhedral phenocrysts of Plagioclase and Amphibole. There is some
biotite. These phenocrysts are in a fine-grained matrix.
Wards 44E7310 rhyolite tuff
Find the subhedral to anhedral SiO2 grains (low interference colors,
white/yellow). I think that at least some of these are cristobalite (the
high T form of SiO2). This is based on the presence of a “tile” structure
seen in some of the grains with crossed Nichols (near the bottom middle
when the number is placed to the right). This tile structure is
characteristic of cristobalite. Why would we expect Cristobalite in a
rhyolite tuff?
34
Find the K-spar (some with Carlsbad twinning). (lath shaped often, grey/white in crossed
Nichols)
The matrix shows compression/flow fabric (trachytic texture) due to alignment of grains.
There are several vugs, or cavities (these areas can be identified under crossed Nichols
because where there are no minerals, the area will be persistently black). These cavities
are filled with vapor-growth crystals that are usually fibrous and radiating. These crystals
likely grew as vapors from the still-hot tuff filtered up through the newly deposited rock.
Wards 44E 7327 Hornblende Andesite
note the porphyritic texture.
phenocrysts include muscovite (clear in plane light, high interference colors under
crossed Nichols and may show birdseye texture, like biotite), plagioclase (lath shape,
polysynthetic twinning), hornblende (equant crystals with orange/yellow interference
colors)
Many of the hornblende grains are altered (chemical reaction in a late stage of eruption or
weathering), and many have reaction rims (corona texture).
round Thin Section 8: Garnet Pseudoleucite Syenite from Magnet Cove Arkansas
This sample is part of a research project that I worked on as a graduate student. You can
use the micrometer slide to support the thin section under the scope.
Notice that it is strongly porphyritic. What does that tell you?
Find the garnet: large brown isotropic crystals (isotropic minerals always appear black
under crossed Nichols). These crystals are cyclically zoned (meaning that they have
bands of different composition that occur concentrically in the crystal). I interpreted this
zoning to be due to periodic drops in pressure in the magma chamber when volcanic
eruptions from the chamber tapped its supply of volatiles. Some of the garnet crystals
have pyroxene poikilitically enclosed, some with the ends of the pyroxenes “ripped off”
These are glomerocrysts, clusters of phenocrysts caught up in the magma during eruption.
Find the hedenbergite (Fe-rich pyroxene): light brown color, orange interference colors.
Find aegerine (green-colored Fe-Na rich pyroxene). The aegerine is a later-stage
pyroxene, which I interpreted began to form after the magma was erupted to the surface
(carrying with it the large garnet and hedenbergite phenocrysts that had grown in the
magma chamber). Notice that the hedenbergite sometimes have aegerine rims, consistent
with this interpretation (corona texture).
35
I interpreted that the change in type of pyroxene was due to the change in pressure during
the eruption and the loss of CO2 from the magma. Notice that both hedenbergite and
garnet have reaction textures of various sorts, indicating that they were no longer stable
in the melt once the eruption began.
Nepheline and Alkali Feldspar: The grey stuff. Nepheline is a low SiO2 mineral. These
grains are strongly altered, indicating that they also were not in equilibrium once the
eruption occurred. Lath shape and simple twinning usually indicates a feldspar grain,
whereas the Nepheline grains tend to be more equant or hexagonal in shape.
Pseudoleucite: Leucite is very easily altered and rarely found. In this rock, you can find
shapes in the thin section where leucite existed as a phenocryst in the matrix. These
shapes are roughly octahedral in shape, indicating that leucite was there. But the mineral
present is no longer leucite (its kind of a grunge of tiny crystals). Thus, it is called
pseudoleucite because it has the shape of leucite but has been altered to a different
mineral.
36
Igneous Rock Petrology
Extrusive Rocks (smaller crystal size, aphanitic)
lava flows (basalt, rhyolite)
pyroclastics (tuff, welded tuff, volcanic breccia)
Intrusive Rocks (larger crystal size, phaneritic)
discordant
concordant
dikes and sills: more rapid cooling usually than larger intrusives (thought puzzle
on crystal size)
plutons, vs countryrock
Cumulate Rocks, subcumulate (crystallized melt between cumulate grains)
Layered Intrusives. Body of magma that crystallizes over time, with different mineral
crystallizes at different times, and the composition of the residual magma changing with
time. Due to localized crystal growth and/or crystal settling, layers of various minerals
develop. (show olivine and chromite layers)
Rock families
Calc-alkaline (typical of subduction and convergent zones, includes granites, andesites,
basalts) Typical minerals: pyroxene, amphibole, mica, quartz, plagioclase and K-spar
Mafic and Ultramafic (typical of divergent zones, mid ocean ridges, includes basalts,
gabbros) Also related to massive flow basalts, such as Deccan Traps or the Columbia
basalt. probably associated with huge mantle plume (like Yellowstone?)
minerals: Ca-plag, pyroxene, olivine
Alkaline High alkalis, low SiO2, (Typical of areas where magmas originate at greater
depths, such as hot spot volcanism in oceanic areas yielding alkali basalts, or continental
divergent areas yielding alkali basalts or syenites to highly alkaline rocks such as
nepheline syenites, kimberlites, carbonatites)
minerals: Na, K feldspars, feldspathoids (nepheline and sodalite, leucite), biotite
Textures:
Euhedral, subhedral, anhedral
Granitic (grains roughly of similar size but “grown together” such that some are euhedral,
some subhedral, some anhedral)
porphyritic (phenocrysts and groundmass)
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Graphic texures (intergrown minerals, usually Qz and Alkali feldspar, where qz lies along
crystallographic planes in feldspar. Probably a near eutectic crystallization and occurs
most often in granites and pegmatites)
myrmekitic (wormlike intergrowths of qz and sodic plagioclase, probably subsolidus,
with eutectic composition)
exsolution: At higher T, there is a solid solution, but as T decreases the range of that
solid solution decreases. This causes two phases to exsolve as T decreases. Often the
exolution occurs along crystallographic planes. Perthite is a common example, Na and K
feldspars form a solid solution at higher T, but not at lower T. So single crystal of Na-K
feldspar exsolves into thin lamellae of Na feldspar and K feldspar.
Draw a phase diagram showing only the solvus curve for Na-K feldspar)
Ophitic: subhedral augite grains enclose plagioclase crystals, indicating concurrent
growth. The tendency of the px to enclose plag is related to growth properties. Often
occurs at intermediate cooling rates (fine to medium grain size), such as occur in dikes or
sills (slower than basalt, faster than gabbro). Sometimes the word diabase is used to refer
to the opposite texture in which plagioclase encompasses augite. Diabase is the name of
the basalt-composition rock with ophitic or diabasic texture.
poikilitic: later crystallizing larger crystals enclose earlier smaller crystals.
Trachytic texture: orientiation of plagioclase laths indicating flow orientation or
compaction.
Coronas: a reaction rim around a crystal indicating that the melt began to react with the
mineral but cooling proceeded too quickly for the reaction to go to completion. E.g early
olivine may be rimmed by later orthopyroxene. As the melt crystallizes olivine, it
becomes richer in SiO2. Eventually, Olivine is no longer stable and pyroxene grows
instead.
vesicles: air pockets formed where gases exsolved from the magma.
Amygdules: ground-water deposited vesicle fillings, zeolites, quartz, calcite, epidote.
macro textures of extrusive lavas
pillow basalts (eruption under water), pahoehoe (ropy, billowy surface, flows
rather than moves as a cored mass of fragmented blocks), aa (clinkers on surface, dense
interior of flow), block lava (shear results in blocks, less rough than aa)
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Igneous Rock Classification:
Norite= Plag + Px, Troctolite = Plag + Ol, ultramafic = more Ol and Px, Anorthosite =
plag, usually cumulate, Peridotite= a field term for Ol+Px.
Quartz-Alkali Feldspar-Plagioclase Feldspar-Feldspathoids (SiO2 deficient)
Phase relations and the justification for the divide based on the alkali/SiO2 contents:
More complex when consider other elements (K), CO2 and water, and especially
pressure.
(from Basalts and Phase Diagrams by Morse)
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Effects of Textures:
Pegmatite
obsidian
Tuff: volcanic sedimentary of ash, glass fragments, variable fusion (welding)
Breccia (volcanic sedimentary, with angular fragments
Aplite sugary appearance, lack of mafic minerals
porphyry (50% phenocrysts)
Special rocks
Carbonatite
spilites: submarine basalt: Book states “All plagioclase has typically been converted to
albite and is usually accompanied by secondary chlorite, calcite, epidote, chalcedony, or
prehnite. Spilites are thought to have been subjected to submarine hydrothermal seawater
alteration...
Serpentinite (altered ultramafic)
Lamprophyres (K-rich, mafic, CO2 rich?)....
Explain the conversion to albite.
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Igneous Rocks and Phase Equilibria
Qz transition, trydimite etc.
then Feldspar transition, sanidine etc
Talk about zoning in crystals. Which way will zoning occur (that is, will outside or
inside of the crystal be more Ca-rich?). Do same thing with Fo-Fa - which way zoned?
Talk about phase rule: F=c-p+intensive variables. Where only T and P vary, this reduces
to F=c-p+2. c=components, p=phases.
Non-solid-solution binary systems: phase diagram overhead.
Pick a couple of compositions and decrease T, showing
first phase to appear on liquidus, zone of freezing, and
encounter of solidus.
Two different phases on the liquidus, depending on the
starting bulk composition.
Last drop of melt will always be at the invariant point
where liquid, phase A and phase B all coexist
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(remember, other invariant point was where three things coexisted). Identify eutectic.
Albite-Qz phase diagram overhead.
What form of quartz if went to even lower T? (high quartz then low quartz) (show other
phase diagram if necessary)
What if at higher Pressure? what would be different? (high Qz instead of cristobalite and
tridymite).
Talk about direction of composition change, whether more Si rich or Si poor. Fractional
crystallization, mention thermal divide
Talk about Ne solid solution, SiO2 dissolved in Nepheline. No Ne dissolved in SiO2.
See Albite-Orthoclase overhead, and also draw a simplified schematic version on the
blackboard. Two solid solution series plus a subsolidus exsolution curve. Note where
various phases occur, including polymorphic transitions. High albite, less ordered Si-Al,
low albite has more ordered Si-Al.
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If slow cooling occurs, microcline occurs in rock. More rapid cooling from higher T
results in orthoclase, or even sanidine.
Bunny rabbit overhead with simplified schematic on blackboard. Effect of pressure
(H2O pressure) on the curve (5 kbar H2O). Explain how this results in a single feldspar
at low water pressure, and two feldspars at high water pressure. Perthite forms when
crystallizes at low P, then cools below solidus curve. If the rock cools at depth with
H2O, 2 feldspars form to start with and perthite does not occur.
Reaction between crystal and melt: pertitectic point, The Fo-Quartz diagram. Also talk
about liquid solvus, and the thermal maximum as go toward MgO, cristobalite becomes
tridymite and quartz at lower T. Figure from Morse, Basalts and Phase Diagrams.
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Three components, get ternary phase diagram, each side of the ternary being a binary
phase diagram. With 4 components it is normally handled by constructing slices through
a tetrahedron (each face of tetrahedron being a ternary phase diagram.
The diagram below is from an assignment that I did for intro to thermodynamics for
geologists.
Also, if consider other variables, such as P, can consider shifts at a variety of pressures.
(diagram from intro to thermo for geologists).
Magma evolution
Equilibrium and fractional crystallization
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Mass balance and chemical equilibrium considered together. Are neither creating nor
destroying elements. But chemical processes divide elements into crystal or melt in
unequal ways, causing the composition of the crystal and melt to deviate from the bulk
composition of the system.
If equilibrium, then composition of both melt and crystal are defined by the phase
diagram. If crystals are somehow removed from re-equilibrating with the bulk system,
then each melt composition is like a new bulk composition, and can get more extreme
changes in melt composition.
Equilibrium “batch” crystallization:
Two equations, one for equilibrium, one for mass balance:
Cs/Cl = D (equilibrium, Cs = concentration of element in solid, Cl = concentration in
liquid, D is the bulk partition coefficient)
(1-F)·Cs + F·Cl = C0 (mass balance, the number of atoms of element don’t change, f =
fraction of original melt remaining, C0 = concentration in the original melt.)
Solving for the two equations simultaneously yields (subst Cs = D·Cl):
Cl = C0/(D-D·F+F)
Fractional Crystallization (Rayleigh fractionation), presuming that each infinitesimal
increment of crystal growth is immediately removed from chemical contact with the melt.
take infinitesimal increments and integrate:
C=C0 F(D-1)
What if D = 1? D>1? D<1? what if melt fraction = 1? what if melt fraction = 0?
Have them do a graph of fractional crystallization (concentration vs fraction crystallized)
Go over this graph, give example puzzles: I give two curves...which one is D=0.05, and
which one is D=0.5? Which one is equilibrium, which one fractional?
Go through calculations of C for specific cases, including fractional and equilibrium
crystallization.
Harker Diagram (a form of variation diagram) for olivine fractionation:
Which elements are compatible in olivine and which are incompatible?
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Equilibrium and fractional melting
Analogous to crystallization, except the composition of a magma changes due to varying
degrees of partial melting rather than due to progressive crystallization of the magma.
Polybaric melting. Melting occurs as a mass of magma and crystals rises, decreasing P,
decreasing melting T.
assimilation
surrounding rock becomes part of the magma. Requires “excess heat”, that is, it takes
energy to melt rock, but if one takes energy from the magma when the magma is at the
liquidus T, then it will begin to crystallize (not melt extra rock). So need to be above the
liquiqus T. This can sometimes happen when a magma rises and P drops, causing the
melting T to drop as well.
Liquid immiscibility
Common evolutionary trends on Earth:
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Tholeiitic, lower fO2, Fe2+ = Fe concentration, often olivine )or other low-Ca
ferromagnesian phase) early crystallizing phase and crystallizes for a long time before get
more Ca-rich phases
Calc-alkaline, higher fO2 (due to more H2O?), Fe3+= fe crystallizes as e.g. magnetite
and is not concentrated. Also, often crystallization of pyroxene and plagioclase more
than olivine.
Phaneritic series from Duluth Complex: Also mention where ferrodiorite and monzonite
fall on the trend as well as gabbro and granite.
Think through the Tholeiitic layered intrusive: Including plag flotation, Ol/pyx sink.
Troctolite (Ol + Plag), Anorthosite (mostly plag), melt trends toward higher Na, K, SiO2,
Fe (enrichment of incompatible elements). Talk about melt overturn (sedimentary
structures sometimes seen), new magma injection, decompression, all of which can
abruptly shift phase equilibria, so get multiple layers with overall trend toward
enrichments stated above.
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Mineralogy and Petrology.
Metamorphic Rocks, Textures, minerals, and grade, Lab #7
Consider each of the following and determine its relative metamorphic grade based on
mineralogy and texture:
80:
87:
77:
74:
Chlorite Schist
Silliminite garnet Gneiss
Slate
Phyllite (compare this also with sample R9 from the introductory lab set)
Consider each of the following, and comment on whether each is likely to be lowP-HiT
(contact metamorphism), HiP-loT (dynamometamorphism), or hiP-HiT (regional
metamorphism), based on texture and mineralogy
75: garnet wollastonite skarn
43A: muscovite schist/gneiss
86: kyanite quartzite
For each of the sample above, and also for the following samples, examine the sample,
try to identify the major minerals (those included in the name of the rock), and think
about metamorphic grade:
47E7224: mica garnet schist
82 talc-tremolite schist
83 graphite schist
80 chlorite schist
81 biotite gneiss (I would probably have called this a schist)
93: actinolite schist
95 hornblende schist
89: augen gneiss
87: silliminite garnet gneiss
97: hornblende gneiss
76: quartzite
73: dolomite marble
T4A: granitoid gneiss (it looks like a granite, but notice the foliation)
R12 (introductory collection): gneiss
R8 (introductory collection): slate
R11 (introductory collection): garnet, staurolite, hornblende schist/gneiss
88: cordierite, anthophyllite skarn
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Mineralogy and Petrology.
Metamorphic Rocks, Textures, minerals Lab #8
Thin Sections:
5: Phyllite In this sample, notice the following:
foliation (the ‘layering’ is due to metamorphism, not sedimentation)
The foliation is defined by alternating bands that are either
quartz-rich (low interference colors)
or
biotite/chlorite/clay rich (darker, pleochroic, higher interference colors)
4: Schist In this sample, notice the following:
large, foliated muscovite and biotite flakes (note the perfect cleavage, birdseye texture
under crossed Nichols, high interference colors, biotite is strongly pleochroic)
Other minerals in this sample include quartz, feldspar (distinguished from quartz
by the presence of cleavage fractures and twinning), garnet (high relief and isotropic-see
below), and an opaque oxide mineral.
6: Gneiss In this sample, notice the following:
Biotite and Amphibole. Both are strongly pleochroic. But amphibole shows two
directions of cleavage, not at right angles, and the crystal goes to extinction NOT
PARALLEL TO THE CLEAVAGE. Biotite has only a single obvious cleavage direction
and extinction IS PARALLEL TO CLEAVAGE.
At least one of the amphibole crystals has very obvious exsolution crystal in it,
oriented parallel to one of the cleavage directions.
Notice that where three quartz crystals meet (look for the low-interference colors
and equant shape), they tend to meet at angles that approximate 120º. This is a texture
typical of metamorphic rocks. Contrast it with the round quartz crystals in sandstone, or
the more irregular quartz crystals in igneous rocks.
There is some feldspar in this sample. Unlike in igneous rocks, these can’t be
identified by their characteristic twinning (twins are annealed away during
metamorphism). You can distinguish it from quartz on the basis of cleavage plane lines,
which the quartz lacks.
There are two “high-relief” minerals. These minerals stand out from the
surrounding minerals (high-relief) because they have a distinctly higher refractive index
(that is, light travels much more slowly through them, thereby refracting the light more).
These two minerals look very similar in plane light, both occurring as small, equant
crystals.
One of these minerals is garnet. Garnet occurs as roughly equant crystals (often
hexagonal-looking), more often associated with the quartz than with biotite or amphibole,
and is isotropic. Isotropic minerals are high-symmetry minerals that do not break light
into separate beams, because of their high symmetry. Therefore, garnet always appears
black under crossed Nichols, regardless of stage rotation.
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The second high-relief minerals occurs often in association with biotite or
amphibole in this thin section. It is not isotropic, but has high interference colors. I think
that this mineral is clinopyroxene, although there isn’t much cleavage apparent.
44 E 7495 Quartzite
In plane light, notice the rounded outline of the original sedimentary sandstone
grains (almost all quartz). Under crossed Nichols, notice the overgrowths on the original
grains, and how three grains tend to meet at 120º (this rock has not been metamorphosed
as extensively as some, and the angles are not as distinctly 120º).
This is a very “clean” quartzite, with only a few rock fragments (original sand
grains made up of clusters of smaller crystals) and muscovite flakes present in addition to
Quartz.
44 W 6173 Marble
Most of this rock is composed of calcite. Notice the rhombohedral cleavage and
twinning pattern (cross-hatched pattern). Calcite has very high interference colors (so
high that most of the color disappears and is called ‘high-order white’ – a sort of grungy
gray-white). In some of the cross-hatched twins you can get hints of the high interference
colors in that you can see glints or red, blue, etc. Notice the 120º intersections at the
corners where three grains meet (characteristic of mono-mineralic metamorphic rocks).
There are a few muscovite grains (high interference colors and birdseye under crossed
Nichols, clear in plane light).
44 E 7386 Kyanite Quartzite
Kyanite is the high relief, medium-to-low interference-color mineral (yellowish
under crossed Nichols). It has prominent cleavage planes, and common poikiloblasts of
quartz (in igneous rocks we refer to poikilocrysts – in metamorphic rocks we refer to
poikiloblasts). Notice the 120º intersections between quartz grains, much better defined
in this higher grade rock than in 44E7495.
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Metamorphic Petrology
Metamorphism: Change in mineralogy and/or texture in the solid state
usually due to changes in T, P, or metasomatism (alteration in the presence of fluids,
usually H2O, that contain various dissolved components, such as Si, Ca, Na, etc.)
T and P are not perfectly independent. Are correlated by the local geothermal gradient.
gradients in top 10000m range from about 0.015 degrees C per meter to 0.03 degrees C
per meter. (or 15 to 30 degrees per kilometer)
Geobarometric gradient, being caused by the weight of the overlying rock, is more
regular, about 3.33 kilobars per 10000 m (or 0.333 kilobars per km). 1kbar = 100
MPascal = 0.1GPascals = 14500psi = 987 atm. (calculate how many degrees per
kilobar).
burial metamorphism (up to 2kbar, 250-300C)
regional metamorphism (usually higher T, at a particular P, than typical geothermal
gradient, consistent with the extra heat typical in the convergent zones where regional
metamorphism occurs.
High-pressure, low T (blueschist metamorphism). lower than typical geothermal gradient
Contact metamorphism (much higher geothermal gradient, near magma intrusion.)
Shock metamorphism (coessite, stishovite, maskelynite on the Moon, other types of
glass)
Boundaries of metamorphism:
low end: first appearance of marker minerals (laumontite, lawsonite, albite, other
zeolites) usually at least 150-200C and 1.5kbars.
high end: below partial melting, which depends on rock type and presence or
absence of water. more felsic with lots of water, melt at lower T (e.g. 650C), more mafic
dry rocks melt at lower T (e.g. over 1000C)
foliation (slaty cleavage, schistosity, gneissocity)
increase in grain size (annealing, reduces surface area/volume)
triple junctions (often tend toward 120 degrees in equisized, monomineralic zones)
blasts (= lump)
porphyroblasts (crystals much larger than matrix)
granoblastic (equisized).
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poikiloblastic (numerous small inclusions)
idioblastic (well-formed crystals)
hypidioblastic (medium crystal development)
xenoblastic (anhedral crystals)
mylonitic (pervasive plastic deformation has caused reduction in grain size (analogous to
brittly formed cataclasis at lower T and P)
blastomylonitic (e.g. augen gneiss) shear define boundaries of eye-shaped shear-bound
graines.
Metamorphic Grade
see overhead or write on board if there is time (makes one think through it rather than
look at it an then think one understands because one saw)
Metamorphic Isograd
Isograd is a surface in three dimensions (a curve on a map) marked by the first
appearance of an index mineral as one goes from lower to higher grade metamorphism in
essentially isocompositional rocks.
e.g. pelitic rocks of Highlands of Scotland, for Barrovian style metamorphism (midlin
geothermal gradient). (Note increasing Ca in plag with grade)
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Chlorite zone: Qz, Chloirite, Muscovite, albite
Biotite zone: Qz, Chloirite, Muscovite, albite, biotite
Garnet zone: Qz, Muscovite, biotite, garnet, Na plag
Staurolite zone: Qz, Muscovite, biotite, garnet, staurolite, plag
Kyanite zone: Qz, Muscovite, biotite, garnet, kyanite, plag (may be staurolite)
Silliminite zone: Qz, Muscovite, biotite, garnet, silliminite, plag
Isograd map of the example above. Remember that the isograds are actually surfaces.
See overhead.
Metamorphic Facies
See overhead.
Metamorphic facies is based on the
idea that the mineral assemblage is
at or near equilibrium and thus is a
function of the T and P at which it
formed and the composition of the
orginal rock. Thus, if two rocks in
different parts of the world formed
at the same T and P, and formed
from the same basic composition
material, they will have the same
mineral assemblages.
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Metamorphic Field Gradients for various facies series superimposed on an Al2SiO5
phase diagram. Consider, which minerals have the highest density? Which the highest
entropy/heat in bonds? What sequence will occur with increasing grade for different
geothermal gradients? Which gradient is like the case in the Scottish Highlands? Which
series will not have andalucite? Which will go from andalucite to sillimite? Which will
go from kyanite to andalucite?
How are the metamorphic field gradients different from geothermal gradients?
The geothermal gradient is what any particular member of the series would have
experienced at its particular location in the earth during metamorphism. Another member
of the series, at a different, perhaps adjacent area, might have experienced a somewhat
different gradient. After these rocks are exhumed to the surface, and we examine them,
we observe a series that have experienced the P-T conditions shown in this figure, but the
P-T field is not necessarily the geothermal gradient present at any particular location.
Other typical minerals: cordierite at lower P series, garnet at higher P series. In general,
higher density minerals at higher P, less hydrated minerals at higher T.
Petrogenetic Grid.
Considers reactions more rigorously than the idea of facies. A petrogenetic grid is
a multicomponent phase diagram (thus valid only for the composition for which it is
designed) showing the phase reactions in P-T space.
See overhead:
Activity: For two metamorphic field gradients, consider the sequence of reactions that
occur, at what T and P particular minerals will appear. Construct an example isograd
sequence based on this analysis. For one gradient, assume both increasing T and P in a
roughly 45 degree angle on the diagram (say, Barrovian series). For the other gradient,
assume contact metamorphism, with T increasing much faster than P (hornfels series).
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55
Mineralogy and Petrology. Review sheet for Exam 2.
Covering Igneous Petrology and Metamorphic Petrology
Igneous Rocks:
Know the important igneous textures, and what they tell us about the rock.
Be able to use the classification diagram for igneous rocks to identify a rock, or to tell
about its properties given its classification.
I will have at least one hand sample to identify (name the rock), and at least one mineral
to identify under microscope (name the mineral). You will only have one minute with
the sample or at the microscope.
Be able to read binary phase diagrams and apply them to both textures and compositional
trends that develop in igneous rocks. (non-complete examples of textures that can be
explained = zoning, perthitic; be able to understand and predict how melt composition
will change with crystallization for any bulk composition portrayed in a diagram.) Be
able to relate the diagram to melt trends during equilibrium melting or crystallization.
Know how to explain fractional crystallization using such a diagram.
Understand magma evolution. Be able to predict trends (on a graph or numerically). Be
able to use the equation giving the liquid composition for any particular degree of
fractional or equilibrium crystallization. Know qualitatively what fractional or
equilibrium melting are, and what assimilation is.
Know about the cal-calkaline and tholeiitic trends. Know their chemistry, the process
that produces chemical changes, and the geological environments where each is more
common.
Metamorphic Rocks:
Know the important metamorphic textures.
Understand the concepts of geothermal gradient, metamorphic grade, isograd,
metamorphic facies. Be able to explain them, what they are, etc.
Be able to map the isograds (based on mineralogy) for a pelitic assemblage of the
Barrovian series, or list the minerals in each isograd if the isograds are given to you.
Be able to read and interpret a petrogenetic grid!!!!
I will have at least one hand sample to identify (name the rock), and at least one mineral
to identify under microscope (name the mineral). You will only have one minute with
the sample or at the microscope.
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Final Exam
Comprehensive, open notes.
85% of questions will be the SAME TYPE (that is analogous questions) to what was on
the regular exams.
15% of the questions will be material on the previous review sheets but not specifically
covered on the exams.
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