CEE 437 Minerals “Lab”
Earth Materials That are Not Minerals
Some earth materials are not minerals as they lack a crystalline structure. Here are some
examples:
Chert/Flint: Like the hydrous form, opal, this is amorphous (i.e. non crystalline) silica.
Cherts are generally forming by dissolution from groundwater. Often this takes the form
of secondary replacement in sediments and sedimentary rocks. A common occurrence is
in the form of nodules or layers that range from a few cm to meter or so scales. The other
major form of silica (not shown here) is organic silica which is formed by sedimentary
processes from the microscopic silica shells of diatoms. This flint was found in a
recently exposed n Denmark. It has been broken possibly by human activity (note
conchoidal fracture surfaces). This was a very significant material for stone-age human
exploitation.
Glass: This is actually an artificial glass, made by in situ melting by a company called
“Geosafe”. Their idea was to make subsurface barriers to ground water flow, i.e. for
contaminant isolation, by melting the soil or rock in place with buried heaters. This is an
example of the resulting material. Volcanic glasses are not much different in appearance.
SHARP SURFACES!! Be careful to avoid cutting yourself
Coal: Coal is one type of organic material that forms rocks but is not a mineral. This
coal is from the Newcastle coal field in suburban Seattle.
Pumice: A frothy volcanic glass. Source: Mt. St. Helens.
Basics of Mineralogy
The collection of mineral specimens here shows the range of crystal forms and habits.
This group illustrates cleavage, luster, hardness, crystal habit and other features useful for
mineral identification.
Included is selenite (or gypsum). Very soft (<fingernail). Composition calcium sulfate,
and a precipitate from seawater into beds (source for plaster), or void fills precipitated
from groundwater. This is nice specimen so be discrete in scratching.
Also here is calcite CaCO3, the main component of limestone, also occurring as
sedimentary beds or precipitated from groundwater. Readily identified by hardness and
fizzing in weak HCl acid. Can be a very minor component in igneous rocks (Africa has a
really weird bicarbonate of soda volcano). Calcite is very important component of
sedimentary cements. A common variant is dolomite (after Dolomite Alps in Italy)
which substitutes Mg for half the Ca. Identical crystal form, but not attacked by weak
HCl acid. Note “dogtooth spar” crystal habit. Cleaves into rhombs (try this on smaller
samples).
A core sample of rock salt. Try the taste test if you trust your fellow student’s hygiene.
This is a few large crystals, though cur circularly by the coring. Note brine inclusions.
Another sample of an evaporite is the red rock with clear cubes. Any guess what the
cubes are. The red material (carnallite) is a salt formed from Mg and K. It is
considerably more soluble than normal NaCl. K-salts like these are the main source of
potash for fertilizers.
Rock salt, anhydrite, and calcite are all soluble to different degrees. Consider the
engineering problems that might arise when rock containing large amounts of these
materials exists near the surface.
Purple sample of fluorite (CaF2). Classic cubic crystal form but cleave into octahedral
(note inside crystals). Contrast with Galena (PbS) which is silvery metallic and grows as
cubes and cleaves as cubes.
Pyrite (FeS2). Three samples. Large cube, and a smaller sample in dodecahedral habit.
Note also sample where pyrite has replaced original material in a scallop shell. Pyrite is
common associate of ore minerals also appearing in plutonic rocks as well as in
sediments that are in chemically reducing conditions (lack of oxygen means Fe ties with
S instead of O).
Sphalerite (ZnS): Massive chunk of ore from Balmat Mine, NY. Note cleavage surfaces
and sub-metallic luster.
Rock-Forming Minerals
There is an assortment of rock forming mineral specimens shown here. Note particularly
the quartz (hexagonal prisms, pyramidal terminations). One only gets nice crystal forms
like this where there is a void in the rock for the crystals to grow into or where the
medium is very soft (clay, mica) compared to the crystal.
Also note olivine (green aggregate of small crystals, also know in gem form as peridot).
A mantle rock also has this name, peridotite.
Two feldspar samples are here. The whitish one is K-or orthoclase feldspar, associated
with the acid or felsic or hi-Si end of the igneous spectrum, and the other is plagioclase
(actually a cut specimen but notice the banding from crystal twinning. The K-feldspar is
from a weathered pegmatite in California.
Amphibole is present in the sample of “Amphobolite”. Use the hand lens to see small
lathes of elongated dark crystals. This with pyroxene is the main ferro-magnesian
mineral families common to silicate rocks. This amphibolite is from a hydroelectric
power tunnel in Quebec.
The dark green, greasy sample is serpentine or serpentinite. It is formed by hydrothermal
alteration of peridotites, and how one usually finds sub-crustal rocks when they have
been sliced up into the crust. This sample is from Ruby Creek, Blewett Pass. It will be
one of our field trip stops.
Quartz is the “glassy” looking mineral occurring well-formed crystals. What is the
symmetry group of this mineral? What can you say about its cleavage properties? What
in the crystal structure would lead to this condition?
The dark brown crystal is garnet. A cubic mineral, here in a dodecahedral habit. Can e a
gem when clear. Also, a common, though minor rock-forming constituent. Can be a
good indicator of temperature and pressure conditions in metamorphic rocks.
Putting it together into rocks
The selection of samples shows how minerals aggregate into rocks. Start looking at the
pegmatite. Pegmatite is a form of granite, a late stage melt in the freezing of a magma
body. As a late stage melt, it concentrated many odd components as well as water, often
forming spectacular mineral specimens. This sample (slightly weathered) shows the
basic granite minerals in a very course form, quite visible to the naked eye. Note the
white mica (muscovite), quartz (glassy appearance, no cleavage) and orthoclase feldspar
(milky, clear cleavage). Make a sketch showing the mineral component. Note if there
any evidence of weathering on this sample.
After looking at the pegmatite take the sample of granite. Granite has the same mineral
components (perhaps with black mica, biotite, and amphibole) as a FeMg component.
Granite from by freezing from magma (melts).
For comparison note the sample of gneiss. It looks a lot like the granite, but it rather than
having a uniform “granular” texture/fabric, it is strongly layered or “foliated” (after folia,
or leaves in Latin). The fabric reveals the metamorphic origin of this rock, formed by
solid-state recrystallization under temperature and pressure.
Also note here two samples of schist. One is Manhattan schist, the bedrock of New York
City, and is a muscovite-dominated mica schist likely derived from continental, High Si
material, like shales, or sandstones. The other is from Icicle Creek in near Leavenworth
in the Cascades near Hwy 2. This rock is a biotite schist with clear garnet inclusions
(porphyroblasts). How does foliation affect the mechanical properties of the rock? How
do these properties relate to the crystal structure of the mineral components?
Two specimens of volcanic rock (one pebble, other a cut bowl). Note “phenocrysts”, or
crystals of feldspar that were already formed when this was blown out of a volcano. The
rapid cooling preserves the crystals but quickly chills the rest into a glass or finely
crystallized ground mass. The texture contrast with granite differentiates the volcanic
versus plutonic origin from melt.
Finally, note a sample of claystone. This has no visible crystalline material in it, but the
analyses indicate that is about equal thirds of very fine grained clay minerals, quartz, and
calcite (both as cements and as sediment grains). This sample is from a depth of 400 m
in a corehole in France. What might happen if it is immersed in fresh water?
Note where each rock specimen might be formed on the figures
below: