CEE 437 Minerals Lab and Discussion
Note: Minerals not in italics are ones you need to know – have some idea their
occurrence and engineering significance.
Rock Forming Silicate Minerals
Quartz (SiO2)
Quartz is an abundant mineral in the shallow or continental crust. It is a survivor –very
stable, hard, has a strong crystalline structure, and it does not weather.
What is the crystal symmetry? ____________________________________________
What is the structure of the tetrahedra? ______________________________________
What type of cleavage does quartz have? ____________________________________
What type of fracture? ___________________________________________________
How does its hardness compare with steel? __________________________________
Note the samples from quartz vein and fracture fillings. If these are from a fracture, what
do the crystals tell you about the opening of the fracture? Note that in cores we can often
say little about how open fractures are, but crystals with good terminations help!
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Note the sample of quartzite – metamorphosed quartz sandstone. How does it differ from
marble? Note that this is an ore sample with a lot of sulfide mineralization, which is
giving the reflections off crystal faces.
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Feldspars
Feldspars are a family of framework silicate minerals with aluminum, and various
combinations Ca, Na, and K. The K feldspars (orthoclase) are monoclinic in crystal form
and usually are creamy white to pink. They are common in granites and continental
rocks. The Na and Ca feldspars are called plagioclase. They are triclinic and vary from
light to dark gray in color. Both have a strong cleavage. The small white sample is
orthoclase feldspar. This is single crystal, how can your tell?
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The larger whitish sample is a pegmatite – a type of granite that has very large crystals
and is a good source of high quality mineral and gem specimens. Look over this sample.
Find a few very large feldspar crystals and make sketch of them.
Note the other minerals in the rock. How can you distinguish feldspar from quartz and
mica?
The other samples here are plagioclase dominated. The smaller dark sample has been cut,
though plagioclase will break easily on its good cleavage. It also has a strong tendency to
“twin”, that is, the lattice can grow in two directions from the same basic pattern.
Twinning in plagioclase results in a kind of striped appearance, where each stripe
represents on twin direction. Find and sketch the twins in this sample.
Some good examples of plagioclase crystals can be seen in the rock bowl. This is an
andesite, a common volcanic rock from island arc or subduction-related places like
Mount Rainier. How can you tell this is volcanic as compared with the pegmatite?
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When did the crystals form relative to the eruption of this rock? ________________
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The pegmatite has some staining from weathering, and there is also plastic tub with some
highly weathered granite in it. What are main weathering products and what survives
weathering best?
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The last rock is a gabbro. This is a dark silica-poor rock that is dominantly plagioclase
and pyroxene, a ferromagnesian mineral. Find and identify the two major components of
this rock. This is a sample from a “granite” warehouse – it is popular (and expensive)
rock for counter tops. It is not a true granite except in the marketing sense.
Other Rock Forming Minerals: Micas, Amphiboles, Olivine, Garnet
Note the large crystals of muscovite (white) mica. The other common mica is biotite
which has iron substitution for some of the cations in the lattice. Biotite dominates the
schist sample in this group.
What silica structure group does mica belong to and how can you tell? _____________
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Note the companion biotite schist sample from Icicle Creek near Leavenworth. Compare
the large garnet sample with the crystals scattered through the schist sample.
Schist is a metamorphic rock and micas are the main metamorphic transformation of
clays. What do clays and micas have in common in crystal structure? How are they
different?
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Describe the state of stress at the time this schist formed? How do the stress state and the
mineral structure affect the anisotropy of rock properties?
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The dark sample is an amphibolite, a rock made up of the ferromagnesian silicate,
amphibole. Amphiboles and pyroxenes are chain silicates made from either double
(amphibole) or single (pyroxene) chains. Both are dark in color and have well developed
cleavage. The main difference is the crystal shape where pyroxenes tend to be short and
amphiboles are long and lathe-like. Which would have the higher silicate content?
Which would have a lower melting point and be more likely to appear in a granite?
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In the volcanic rock in this group, what are the light colored crystals? Which group are
the dark crystals like to belong to?
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Olivine
Olivine ((Fe,Mg) SiO2) is the ferromagnesian mineral most associated with the mantle
and oceanic crust. It is green to black in color, and it has a cubic composition and no
cleavage. There is a small sample here from a cavity in a basalt lava flow. It has the gem
name peridot and it is the main constituent in a mantle-derived rock, peridotite. Rocs that
have olivine composition are very susceptible to alteration, and most large masses of such
rocks are usually altered hydrothermally at depth to serpentine or serpentinite. This rock
is not common in the crust, but it does appear in current (Washington) or former (Central
California) subduction zone areas.
What silicate structure group is olivine? How might this affect the cleavage properties?
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Evaporite Minerals and Carbonates
Fluorite (CaF2)
I put fluorite here more for its illustration of crystal form. Fluorite is commonly purple
though it can be other colors. It is an important industrial mineral and can occur in
masses suitable for mining, especially in Missouri. What sort of anisotropy of properties
will this mineral have? How can you tell this?
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Calcite (CaCO3)
Calcite is one of the most important and abundant minerals in the near surface regions of
the crust. It has an important variant, dolomite, where Mg substitutes for half of the Ca.
Dolomite arises from a reaction of calcite with groundwaters especially early after
deposition. The two mineral are indistinguishable except for acid susceptibility. A
common test for calcite is to place a drop of dilute HCl on sample look for fizzing.
Calcite is relatively soft, will scratch with a knife but not a fingernail. Calcite is the main
component of limestone and marble. Limestones are largely the product of biological
activity, the rock being made mainly of shell and other biologic hard-structure debris. ,
and marble is the re-crystallized, metamorphic rock derived from limestone.
Calcite readily dissolves and re-precipitates in groundwater, and thus is extremely
important as a fracture filling or vein filling material. It is a common cement in
sandstones and other sedimentary rocks. It is rare in igneous rocks, though only as the
very last crystallization stages of water-rich melt fluids.
Calcites and to a greater extent the evaporite minerals, are very important from an
engineering standpoint, both as raw material sources (limestone/marble -> cement), but
also for the results of groundwater dissolution in cave and sinkhole formation. Sinkholes
and caves may be common in limestone-dominated terrains. They are very important
anywhere evaporite units come near the surface. Evaporitic rocks (salt, gypsum) are
seldom seen at the earth’s surface except in very arid environments. Where they would
occur in sedimentary sequence, the terrain is dominated by collapse and dissolution
features.
What is the symmetry of calcite crystals?
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Describe the cleavage (feel free to whack at smaller samples) ______________________
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Note the sample of “dogtooth” spar crystals. As with fluorite, many minerals have one
growth habit and a different cleavage plane. For fluorite, the growth habit is cubic, while
the cleavage is octahedral.
In this same group we have a good specimen of anhydrite (same as gypsum with a
variation of water content). The distinction has engineering importance as subsurface
CaSO4 tends to be anhydrous (anhydrite), and the hydration process gypsum near the
surface results in an expansion that can cause heaving and other interesting effects.
Try scratching a non-conspicuous area with your fingernail. How is it different from
calcite?
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There are three rock samples made of calcite, two limestones and one marble. The
marble is a limestone that is recrystallized (metamorphosed) under temperature and
pressure. I collected this sample a few weeks ago from next to a granite intrusion that
had “cooked” some limestone to marble. How can you tell which rock is the marble?
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Describe the evidence for a biologic origin for one of these rocks.
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The third rock sample is lime sandstone (still called limestone) but it is made of sandsized lime debris probably from what was once a coral reef in a tropical setting.
Limestones deposits today in the parts of the world that are warm enough for calcite to
remain solid and not dissolve – it has a reverse solubility with temperature hence corals
and other major accumulations of limestone only occur in the tropics. Where should
limestones be forming today in the US?
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Halite, Anhydrite, Sylvite, Carnallite)
Evaporites are those earth materials that form by evaporation usually of sea water or
concentrated saline lake waters. The evaporite minerals in order of solubility are gypsum
(or anhydrite, CaSO4), halite or rock salt (NaCl), and the potash salts, sylvite and
carnallite. The last of these are relatively less common and only form in the most
extreme Evaporitic conditions.
Compare the anhydrite to the core sample of rock salt (halite, NaCl). What is the
cleavage geometry of halite? How is it different from gypsum? Note the voids inside the
crystals. These are fluid inclusions. What is their geometry?
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To complete this set there is also a sample of sylvite (KCl). Sylvite is the main source of
potash for fertilizer and other industrial applications. Sylvite looks like rock salt but with
a pink tint. The plastic tub contains what was once a nice sample of carnallite (a K, Mg
chloride salt). These potassium salts are the most soluble of the evaporite minerals. The
plastic tub has what is left of what was a very nice sample carnallite last year. I collected
it from a potash mine in Saskatchewan. Carnallite is deliquescent – it absorbs water from
air. Unfortunately I did not unpack it last year, and it absorbed a lot of water with air
space to evaporate, and the sample basically dissolved itself. The newspapers it was
wrapped in the were soaked and the boxed has some puddles of water. Note the clear
clusters of halite crystals among what is left of the carnallite. These would have
crystallized first, and the surrounding pore volume would have been filled with the
carnallite.
All these have salty taste (try if you’re not squeamish about sharing samples. The
potassium salts taste like rock salt but with some bitterness. The carnallite taste rather
bad in fact.
Sulfides and Other Ore Minerals
Sulfides (Pyrite, Galena, Sphalerite)
There are three sulfide ore mineral in this group – pyrite (FeS2), galena (PbS) and
sphalerite (ZnS). All are major ore mineral that have the same crystal symmetry. What
symmetry group is that?
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Of these, the most important is pyrite, which is a very common mineral throughout the
earth’s near surface. Pyrite is the form of iron that appears in any reducing environment,
i.e. oxygen-free. Note the fossil scallop that had pyrite substitute for its original shall
material.
Pyrite is very important for engineering as it weathers readily on exposure to air and
oxygenated water forming sulfuric acid. This is problem not only for mine tailing that
are often rich of the relatively worthless “fool’s gold”, but also in an rock material used
in construction. Pyrite can also be common in some black shales that were deposited
under reducing conditions in deep, oxygen-poor water.
The oxidized form of iron are hematite, limonite, and magnetite. The sample here is a
metamorphosed iron ore from Sweden consisting of magnetite and hematite. Most iron
ore is either sedimentary or in metamorphic rocks derived from those sediments. We
know that conditions in the earths surface were much more favorable in the past for iron
deposition at the earth’s surface. We also know now that iron deposition is greatly
enhanced by iron-fixing bacteria. Bright red slimes sometimes can be observed in
tunnels, mines, and open cuts are colonies of these bacteria doing what they do best.
The final samples are copper ores, malachite (copper carbonate) and unusual native
copper, which was the ore common to northern Michigan. The other common copper ore
(see the quartzite sample) is chalcopyrite, or (CuFe)S2, which looks like pyrite by with a
more bronze and less brassy color.
Not Minerals at All
This last group has a number of common earth materials that are not minerals at all. The
group contains a glass (be very careful – it can cut!!), coal, and a chert. Chert is an
amorphous form of silica. How does it fracture?
This one was collected from a beach in Denmark and was among many other shards that
may have been stone-age tools.
The coal sample is from Coal Creek in what is now the urban Eastside of King County.
This resource played a major role in the early economic development of this area.
The glass (and I mean it really can cut – it cut me putting this stuff together!), is artificial.
It is from “Geosafe” which was an attempt to immobilize contaminants by in situ melting
or “vitrification” from the Latin root for glass. Why is this not a mineral besides being
artificial?
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