Download lab 2: silicate minerals

GEOLOGY 640: Geology through Global Arts and Artifacts
LAB 2: SILICATE MINERALS
FRAMEWORK SILICATES
The framework silicates quartz and feldspar are the most common minerals in
Earth’s crust. Quartz (SiO2) is one of the few common minerals that is harder
than a streak plate. It may display numerous colors (purple=amethyst; pink=rose
quartz; brown=smoky quartz; yellow-orange=citrine). It may form long hexagonal
crystals but lacks cleavage, and instead breaks along irregular, curving surfaces
(conchoidal fracture). In many cases quartz forms masses of microscopic
crystals (e.g., chert, flint, chalcedony) that still maintain the hardness and
conchoidal fracture of quartz. Banded chalcedony is called agate, whereas
reddish chalcedony is called carnelian (bloodstone).
Plagioclase is a group of feldspar minerals that have complete solid solution
from NaAlSi3O8 (albite) to CaAl2Si2O8 (anorthite). Na-rich plagioclase tends to
white in hands sample, whereas Ca-rich plagioclase tends to be dark grey.
Twinning is the intergrowth of two or more crystals in a symmetrical fashion by
the sharing of lattice points in adjacent crystals. In plagioclase, the most common
twins are planar and repeated (polysynthetic twinning), resulting in the striations
that are characteristic of plagioclase in hand-sample. Twinning tends to be better
developed in Ca-plagioclase minerals. Ca-rich plagioclase (labradorite and
anorthite) may also display iridescent colors (mostly blue). Iridescent albite is
rare and is known as the semi-precious gem moonstone.
Microcline (KAlSi3O8) is the most common alkali feldspar. It is similar to
plagioclase in most of its optical properties (hard, blocky, 2 cleavages at 90°). It
is most commonly salmon pink, but may also be green (amazonite) or white.
Microcline lacks striations but commonly displays subparallel, irregular, wispy
lines.
Examine samples of plagioclase (albite and labradorite) and microcline. Note the
physical characteristics described above. Be sure to be able to recognize the
striations of plagioclase and the wispy lines of microcline.
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Describe the properties that allow you to differentiate between feldspar, quartz,
calcite and dolomite.
SHEET SILICATES
Micas are a group of minerals that have perfect basal cleavage and are capable
of splitting into thin laminae along their perfect basal cleavage. Although they are
monoclinic minerals, they commonly form hexagon-shaped crystals. Micas are
hydrated aluminum silicates. Micas are composed of sheets of silicate
tetrahedrons that are composed of inter connected six membered rings. These
rings are responsible for mica's typical six-sided crystal shape. Muscovite
(KAl2Si3AlO10[OH,F]2) and biotite (K[Mg,Fe2+]3AlSi3O10[OH,F]2) are the two most
common micas.
Clays are a group of hydrous sheet silicates that minerals that are fine-grained
(usually microscopic). The mica-like form is usually apparent only at high
magnification (see photo below) Most clay is produced from chemical weathering
of rocks. For example, kaolinite Al2Si2O5(OH)4, the clay that is used to make
porcelain, is formed from the hydrolysis (reaction with acid and water) of feldspar.
Bentonite (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n is a swelling clay, that is it
absorbs water and expands when wet. Its absorptive properties make it useful
today in items such as cat litter and absorbants for chemical and oil spills. It is
derived
from
the
weathering
of
volcanic
ash.
Glauconite
([K,Na][Fe3+,Al,Mg]2[Si,Al]4O10[OH]2) is a green clay that is found as rounded
aggregates of very fine grained scaly particles. It is generally indicative of
deposition on a continental shelf with slow rates of sediment accumulation. Talc
[Mg3Si4O10(OH)2], is a very soft, pearly clay with a waxy feel. Rocks composed of
massive talc may be referred to as soapstone or steatite.
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Scanning Electron Microscope Photo of Kaolinite
Describe the properties that allow you to differentiate between kaolinite, chalk
(calcite), and alabaster (gypsum).
Describe the properties that allow you to differentiate between talc and gypsum.
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CHAIN SILICATES
Amphiboles are a diverse group of generally dark-colored rock-forming minerals
composed of double chain SiO4 tetrahedra. Amphiboles are hydrous minerals
and contain Fe and/or Mg. Most amphiboles contain Ca, Na or K as well.
Amphiboles have two cleavage planes at around 120 degrees, and crystals tend
to be long (prismatic to acicular). Hornblende (Ca2[Mg,Fe,Al]5[Al,Si]8O22[OH]2) is
the most common amphibole; this black prismatic mineral is a common
constituent of both igneous and metamorphic rocks. Jade is a rock composed in
large part of the rare amphiboles jadeite and/or nephrite. Jade is a very strong
rock due to the intergrowth of these acicular green amphiboles.
The pyroxenes are an important group of single chained silicates, and are
common in igneous and high-grade metamorphic rocks. The single-chain
structure leads to the prismatic to fibrous character of pyroxenes, and the
characteristic cleavage angle of the pyroxenes at nearly 90o degrees. Pyroxenes
are similar to amphiboles in structure, composition and appearance. However,
pyroxenes tend to form stubbier crystals and tend to have a greasier luster than
amphiboles.
The
most
common
pyroxenes
is
augite
(Ca,Mg,Fe,Na)(Mg,Fe,Al)(Si,Al)2O6, which is dark green. Bronzite (Fe,Mg)SiO3 is
a pyroxene that is brown, and exhibits a reflective, bronze-like appearance on
polished surfaces.
Examine samples of amphibole and pyroxene. Note the physical characteristics
described above. Note the variations in shape, cleavage, and luster.
Describe the properties that allow you to differentiate between the three most
common dark silicate minerals: biotite, hornblende, and augite.
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RING SILICATES
Tourmaline Na(Al,Fe,Li,Mg,Mn,Cr)3(Al,Mg,Fe,Cr)6[(Si,Al)6O18](BO3)3(OH,F)4 is
the most common ring silicate. The many possible compositions of tourmaline
allow for a wide variety of colors. Cross-sections of tourmaline have a rounded
triangular shape.
Describe the properties that allow you to differentiate between the two prismatic
minerals tourmaline and hornblende.
ISOLATED TETRAHEDRON SILICATES
Olivine is not an official mineral name itself, but is the name for a solid-solution
series between the Fe-rich end members, fayalite (Fe2SiO4) and the Mg-rich end
member forsterite (Mg2SiO4). The two minerals form a series where the iron and
magnesium are substituted for each other without much effect on the crystal
structure. Olivine is a common component of mafic (e.g., basalt and gabbro) and
ultramafic igneous rocks (e.g, dunite).
Garnet is also the name for a group of related minerals that exhibit a wide range
of solid solution. Most garnets can be described by the chemical formula
(Ca,Fe,Mg,Mn)3Al2(SiO4)3. Garnets tend to be granular and exhibit a complex,
many-faced, equant form, and are most common in metamorphic rocks. Garnets
lack cleavage. Garnets exhibit many different colors, but the most common color
is a deep wine-red.
Examine samples of olivine and garnet. Note the physical characteristics
described above.
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Describe the properties that allow you to differentiate between the two common
pale green minerals olivine and diopside.
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