Download mineralogy - West Virginia University

Dr. Helen Lang
Dept. of Geology & Geography
West Virginia University
FALL 2015
GEOLOGY 284:
MINERALOGY
Metamorphic Rocks and
Minerals
Metamorphism means Change
Rocks Change as Conditions
Change
• When changes in conditions are not too
dramatic, we call changes in sedimentary
rocks diagenesis (up to about 200oC)
• When new minerals, which are never
observed to form at Earth’s surface, form,
we call the changes metamorphism
Metamorphism is:
Changes in a rock’s mineralogy,
texture or composition without
melting (in the solid state)
Any kind of rock can become a
Metamorphic Rock
• Parent rock of a metamorphic rock is called its
protolith
• Meta-igneous rocks
– Originally igneous, have been changed
• Meta-sedimentary rocks
– Originally sedimentary, have been changed
• Poly-metamorphic rocks (meta-metamorphic rocks)
– Already metamorphic, changed again
What causes Metamorphism?
• Increase in temperature (T)
– T increases with depth in the Earth
• Increase in pressure (P)
– P increases with depth in the Earth (from
the load of overlying rock)
• Deformation (changes rock texture)
• Movement of fluids (like water vapor)
Metamorphic Minerals
The two most important factors
controlling mineralogy of
metamorphic rocks are:
1.
2.
the composition of the rock
(the available ingredients)
the grade (intensity) of
metamorphism.
We account for rock composition
by grouping rocks into general
compositional classes
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•
•
•
•
•
Pelitic rocks
Psammites
Mafic rocks
Carbonates
Ultramafic
Granitic
clay-rich sediments, shales
sandstones
basalts and gabbros
limestones
mantle rocks
granites
We’ll consider important minerals in
each of these rock types separately
• Psammites
–
–
–
–
–
–
meta-sandstones
start with Quartz and Feldspar
metamorphic rock has Quartz and Feldspar
coarser grain size
rock may be foliated (layering caused by deformation)
called Quartzite or Quartzofeldspathic Gneiss
Not much change with metamorphism
Metamorphosed Granites
• Start with Quartz, Feldspar and Micas
• Metamorphic rock has Quartz, Feldspar, and
Micas
• Rock may be foliated (gneissic layering)
• Metamorphic rocks called granitic gneiss
• Again, not much change with metamorphism
Metamorphosed Limestone
• If you start with just Calcite and/or
Dolomite, not much change
• Grain size increases: Limestone  Marble
• If protolith limestone has quartz silt or clay
minerals, many more minerals are possible
• Dirty limestones display interesting changes
with metamorphic grade
Metamorphosed Shales, Pelitic Rocks
• Sedimentary protolith has Quartz silt and
Clay minerals
• Clays contain K, Fe, Mg, Ca, and especially
Al in addition to SiO2 and OH (from water)
• These elements can make many new and
interesting minerals
• Their mineralogy also changes dramatically
with metamorphic grade or intensity
• We’ll spend some time on these!!
Some Minerals we already talked about
occur in Pelitic Metamorphic Rocks
What silicate subgroup?
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•
•
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Quartz
Plagioclase
Muscovite
Biotite
What are the new and interesting minerals?
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•
•
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Garnet
(Ca,Mg,Fe,Mn)3Al2Si3O12
Staurolite
Fe2Al9Si4O23(OH)
Andalusite
Al2SiO5 [Al2OSiO4]
Sillimanite
Al2SiO5 [Al2OSiO4]
Kyanite
Al2SiO5 [Al2OSiO4]
Notice the essential Al, which comes from
clay minerals, like kaolinite, Al4(Si4O10)OH8
• All of these minerals are isolated tetrahedral
silicates
• All are porphyroblastic (big, visible crystals),
except sillimanite.
Garnet Structure
• Garnet
(Ca,Mg,Fe,Mn)3Al2Si3O12
• isolated tetrahedral silicate
(SiO4)-4 tetrahedra
(AlO6)-9 octahedra
Ca2+, Mg2+, Fe2+ or Mn2+
Garnet Properties
• Commonly dodecahedral,
almost always equant
• H=7
G=3.54
• Resinous luster
• Color varies with composition
– Fe, Mg garnets pink to red
– Ca-rich garnets are colorless,
orange, yellow, brown or green
dodecahedron
• Optical
– hi relief
– isotropic
More Garnet Examples
Garnet trapezohedra (another
equant, many-sided form)
Gore Mountain Garnet Mine,
NY, garnet mined for sandpaper
Garnet Porphyroblasts in thin section
Euhedral
garnet
porphyroblasts in
PPL
Staurolite
• Fe2Al9Si4O23(OH)
• Isolated tetrahedral silicate
• Stubby Prisms, commonly
euhedral
• Simple, penetration crosstwins are common
• Resinous luster
• Brown color
• H=7-7.5
G=3.75
Twinned Prismatic
Staurolite
Optical Properties of Staurolite
•
•
•
•
•
High relief
Low birefringence, maximum 1st order yellow
Clear, yellow or light brown color
Pleochroic
Commonly exhibits a sieve-like texture due to
abundant quartz inclusions, like Swiss cheese
Staurolite in a Thin Section
The Aluminosilicate Polymorphs
Al2SiO5
• Andalusite
• Sillimanite
• Kyanite
All are isolated tetrahedral silicates (Al2OSiO4)
These minerals are very important for telling
pressure during metamorphism
View Andalusite Movie
http://socrates.berkeley.edu/~eps2/wisc/geo360/andalusite.mov
Pressure
The Al2SiO5 Phase Diagram
The triple point
~500oC
~4 kilobars
(~4000 atm,
~12 km depth)
Kyanite
SG=3.60
Sillimanite
SG=3.23
Andalusite
SG=3.18
Temperature
Kyanite Properties
• Blue, bladed crystals
(can be clear or white)
• H=5 along blades, H=7
across blades (it’s anisotropic!)
• SG = 3.60
• One perfect cleavage
• Luster vitreous, pearly
Kyanite Examples
Kyanite Optical Properties
• Typically colorless in thin section, may
be blue and pleochroic
• High relief, low birefringence, but
higher birefringence than andalusite
• Monoclinic, inclined extinction
• Bladed shape with one perfect and one
good cleavage
Kyanite in PPL
and XPL
“Virtual Geology” Project
Glazner & Ratajeski UNC
Pressure
The Al2SiO5 Phase Diagram
The triple point
~500oC
~4 kilobars
(~4000 atm,
~12 km depth)
Kyanite
SG=3.60
Sillimanite
SG=3.23
Andalusite
SG=3.18
Temperature
Andalusite
Low pressure form of Al2SiO5
•
•
•
•
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Luster vitreous
Color pink or white
H=7.5, G=3.18
Stubby, square prisms
May show a crossed pattern of graphite
inclusions, called Chiastolite
• Optically lower relief and lower
birefringence than kyanite
Andalusite
Square prisms in hand specimen
Andalusite
Square prisms in hand specimen
Chiastolite cross –
graphite inclusions get
concentrated along
diagonals
Sillimanite Properties
(high T polymorph)
• Acicular to thin prismatic
• Clear, colorless and vitreous
• May form mats of fine fibers, called fibrolite
• Optically, sillimanite has high relief,
moderate birefringence, and parallel
extinction
• Clear needles, thin prisms or fibers
Sillimanite in Outcrop
Rarely visible on outcrop scale
Sillimanite
in thin
section
PPL
XPL
Pressure
The Al2SiO5 Phase Diagram
The triple point
~500oC
~4 kilobars
(~4000 atm,
~12 km depth)
Kyanite
SG=3.60
Sillimanite
SG=3.23
Andalusite
SG=3.18
Temperature