Lab 4 - Geology

Geo 306 lab 03
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Name __________________________________________________
Physical and Optical Properties of Sheet Silicates, Ring Silicates
Goals for this lab:
1. Learn essential properties for identifying these minerals in hand specimen: serpentine, kaolinite,
talc, muscovite, biotite, chlorite, beryl, tourmaline.
2. Learn essential properties for identifying these framework silicates in thin section: talc, muscovite,
biotite, chlorite, tourmaline.
Lab lecture topics
1. Finding each mineral in thin section.
Lab groups: You will be sharing microscopes, and can work together, but should turn in individual
work on this lab.
Introduction
The sheet silicates are a very common and important group of crustal minerals. They all contain hydrogen
(some more than others), so they are not stable at very high temperatures. These minerals are ubiquitous in
many metamorphic rocks, and of course clays are extremely common in the near-surface environment.
Muscovite and biotite are important components of felsic to intermediate plutonic igneous rocks, and
serpentine is commonly found as an alteration product of ultramafic rocks in our area (c.f., Swift Creek
Landslide).
It would be wise to indicate on your notes something about the crystal structure (T-O, T-O-T, etc.).
The ring silicates are not systematic in their occurrence.
Serpentine
Hand sample notes:
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Rarely forms large or euhedral crystals. Most commonly found nearby as product of hydrothermal
metamorphism of ultramafic rock.
• Hardness and color are very useful, but luster is a key tool as well.
• Similar: green talc, nephritic actinolite, chlorite
Kaolinite
Hand sample notes:
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Essentially never forms visible crystals.
Luster is distinctive.
Breathe on it, then smell it – useful.
Similar: Chalk (calcite), diatomite (quartz)

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Talc
Hand sample notes:
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Hardness and luster are key.
Color is somewhat variable, but generally light.
Often occurs mixed with other minerals, so bulk hardness can be misleading, but soapy feel is useful.
Similar: serpentine, muscovite
Thin section notes:
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NC022E is dominated by two minerals: actinolite (an amphibole) and talc. Talc is the mineral with one
cleavage. It is nearly indistinguishable from muscovite in thin section (one reason hand specimen
analysis is as important as thin section).
• Talc, like muscovite and biotite, has a texture in XP called “bird’s-eye extinction”. This texture is
produced when the surface of the thin section is ground down. During this process, the surface get
roughened, and these irregularities do not go extinct in the same orientation as the rest of the grain.
What you see is a speckled appearance as the grain goes extinct.
• Some of these thin sections are not the correct thickness, so be careful.
• Similar: muscovite
Muscovite
Hand sample notes:
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Habit and cleavage, together with color, are key.
Similar: biotite, talc
Thin section notes:
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Muscovite, together with biotite and talc, has a distinctive appearance in XP near extinction. Find a
grain showing this texture, called “bird’s-eye extinction”, and sketch it.
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Micas often look very different when you are looking on a cleavage plane versus looking at a cut
through the cleavage planes. The bird’s-eye texture looks different as well. Find one grain of each
type and describe the differences.
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Muscovite and the other micas are easily deformed. Just as with quartz, this deformation causes
undulatory extinction.
• PM026A has lots of small muscovite grains (and a few large biotite grains). Note the bright secondorder
colors.
• MBr38 has some very fine muscovite grains.
• Similar: talc, biotite

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Biotite
Hand sample notes:
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Looks essentially like muscovite, but darker-colored.
Similar: chlorite, hornblende, muscovite
Thin section notes:
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Looks essentially like muscovite, but darker-colored, and pleochroic. Note that when the biotite
cleavage is oriented N-S in PP, it can looks nearly colorless.
• Has bird’s-eye extinction.
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MBr38 has lots of biotite, aligned generally parallel.
PM026A has a few large biotite grains.
MBr42 has large biotites with pleochroic haloes. These are small dark circles in PP light, some of
which have tiny clear grains in the center. These grains have radioactive elements which, when they
decay, produce damage in the surrounding biotite, disrupting the crystalline structure and turning into a
semi-amorphous substance. Those without evident tiny central grains have a cut through the halo
which missed the central grain. In this rock, the central grains are zircon, but other minerals can also
cause these haloes. Sketch a grain with pleochroic haloes.
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SN14 has two colored pleochroic minerals. Amphibole lacks bird’s-eye extinction, while biotite has
bird’s-eye extinction. Be sure you can tell the difference.
• Examine I-S-21 with PM026A in XP at low power. Look at the interference colors of each. Note that
these minerals have similar birefringence, but they looks quite different in XP. This is because the
interference color in tinted/obscured by the color of the mineral (i.e., in PP).
• Similar: chlorite, hornblende, muscovite, tourmaline
Chlorite
Hand sample notes:
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Very often too fine-grained to observe individual crystals, but color and habit/state of aggregation can
often be discerned in bulk samples.
• Green to very dark green colors are the rule.
• Similar: biotite, talc, serpentine
Thin section notes:
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Pleochroic green colors.
One cleavage plane.
Very low interference colors. Note that chlorite has anomalous interference colors. This means that
although the interference color maybe first-order gray, it appears brown (if Mg-rich) or blue (if Fe-rich).
Generally the hue of this weird color doesn’t fit anywhere on the interference color chart, and it doesn’t
decrease near the edges of the grains – that’s what makes it anomalous.
• SN14 and 36AC have a very common occurrence of chlorite, as an alteration product of biotite. Look
for pleochroic green layers in the brownish biotite grains. Note that the cleavage of the chlorite is
parallel to that of the biotite. This alignment of crystal structures is called epitaxy
• Similar: biotite, muscovite, hornblende, talc

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Beryl
Hand sample notes:
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Columnar hexagonal crystals with imperfect basal cleavage and conchoidal fracture otherwise.
Mainly found in granitic pegmatites (intrusive rocks with very large crystals).
Typical color is pale blue or blue-green, but deep green beryl is called emerald, blue is aquamarine,
and other colors have other names. Makes a good gemstone due to its hardness
• Similar: apatite, quartz
Tourmaline
Hand sample notes:
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Very strongly colored; typically nearly black in hand specimen (“schorl” variety). Gem varieties can be
more transparent.
• If Li is dominant, then it can be pink (“rubellite” or “elbaite”).
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• Similar: hornblende, augite
Always prismatic to acicular, with distinctive cross-sectional shape.
Common in pegmatites and metamorphic rocks. Main residence for boron in the crust.
Thin section notes:
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Strongly pleochroic in all cases.
W89 is full of tourmaline. We saw this in Lab 2 – note the difference in pleochroism between cuts
perpendicular to the c axis and those parallel to the c axis of this hexagonal mineral. Look for the
distinctive cross-section.
• Similar: hornblende, biotite, chlorite

Geo 306 lab 04
Name __________________________________________________
Questions for Sheet/Ring Silicates
1. What is the best (although still not 100% certain) way for you to tell muscovite from talc in thin section?
(Note that you cannot determine 2V until you take Geology 407) _________________________________
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2. Both quartz and beryl can be blue in hand specimen (although blue is pretty rare for quartz), and they
occur in similar rocks. What is the best way to tell them apart in the field? (“In the field” is intended to make
you think about tools for making measurements) ______________________________________________
_____________________________________________________________________________________
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3. Muscovite, biotite, and talc share a distinctive property in thin section that no other common minerals
display. Sketch it:
4. There are four rocks for you to inspect. Determine which (if any) of this lab’s minerals are found in each
rock:
306X3
306X4
306X5
306X6