Minerals in Thin Section
Terms
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- microcline
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KAlSi3O8
*scotch plaid
*lamellar intergrowth
*one/two cleavages - plagioclase
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CaAl2Si2O8 - NaAlSi3O8
*polysynthetic twinning
*clear color - quartz
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SiO2
*colorless
*lack of cleavage/alteration/twinning
*undulatory extinction! - beryl
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Be3Al2Si6O18
*hexagonal basal sections
*often contains fluid inclusions - cordierite
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(Mg,Fe)2Al4Si5018
*alters!
*not perfect twins
*pseudohexagonal basal sections - Talc
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Mg3Si4O10(OH)2
*high interference colors
*platy masses
*fibrous aggregates - Biotite
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*brown to yellow,red or green mica
*one excellent cleavage
*shows bird's eye extinction - Muscovite
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*clear mica
*2nd order interference colors
*bird's eye extinction - Chlorite
- In general, keys to identifying chlorite are its lack of pronounced features, habit, (normally) colorless or greenish color, low order or anomalous interference colors. Chlorite's features generally make identification straightforward.
- Stilpnomelane
-
*dark brown
*pale yellow
* or green mica- like
*strong pleochroism
*shows 2 cleavages - Chloritoid
- Keys to identifying chloritoid are its relatively high relief, single cleavage, greenish color, and low-order or anomalous interference colors. If visible, an "hourglass" structure, due to zoning, is diagnostic.
- Staurolite
- Keys to identification are staurolite's yellow color and pleochrosim, and its occurrence in medium grade pelitic rocks.
- Kyanite
- Keys to identification are high relief, bladed habit, good cleavage and first order interference colors.
- Andalusite
- Keys to identification are high relief, low birefringence and parallel extinction.
- Silimanite
- Keys to identification are high relief, needle-like, fibrous or bladed habit, characteristic square cross sections with one diagonal cleavage. It is clear and shows upper 2nd order interference colors.
- Garnet
- Garnet is one of the few common isotropic minerals. It is generally colorless or has a pale tinge, often irregularly fractured, and has high relief.
- Hornblende (actinolite)
- Strong brown or green coloration and pleochroism, and 60o-120o cleavage angles identify hornblende. Actinolite is similar, but its color ranges from light green (Fe-poor varieties) to dark green (Fe-rich varieties).
- Epidote
- These minerals may be hard to identify if present as small accessory grains. When larger, identification is simpler. High relief, anomalous interference colors or (for epidote) grains showing many interference colors, are keys to identification. Relief and birefringence increase with increasing Fe-content.
- Tremolite
- Tremolite is identified by its bladed habit, amphibole cleavage, clear to pale green color and upper-first to low-second order interference colors.
- Olivine
- Olivine is in many ways similar to clinopyroxene. The keys to identifying olivine are its high birefringence, lack of cleavage (but often having fractures), and alteration.
- Spinel
- ·Appearance and habit - Spinels are isotropic, have high relief, and show no cleavage. They may form octahedra but, more commonly, are anhedral.
- Clinopyroxene
- The keys to identifying calcic clinopyroxene are normally its high relief, pale green (sometimes clear or light brown) color, middle second-order interference colors, and near 90o cleavage seen in some views. Distinguishing the different pyroxenes and olivine can sometimes be difficult.
- Orthopyroxene
- Orthopyroxene is in many ways similar to clinopyroxene. The keys to identifying orthopyroxene are its relief, usually pale green (sometimes pleochroic to pink) color, low-order interference colors, and near 90o cleavage seen in some views.
- Sphene
- Titanite is one a few minerals with very high relief and very high order interference colors. When visible, its sphenoid or wedge-shaped crystals are diagnostic.
- Rutile
- Keys to identifying rutile are its strong yellowish to reddish brown color, high relief, and extreme birefringence.
- Graphite
- Graphite is one of several opaque minerals that can be difficult to distinguish. In reflected light it appears silvery and metallic, occasionally brownish gray. It may appear similar to magnitite or other silver-colored oxides, but generally has lower reflectance. Perhaps the best way to distinguish it from other opaques is that is sometimes has a sheety habit and cleavage.
- Calcite
- These two carbonates are generally colorless, have extreme birefringence, and show one or two sets of polysynthetic twins that may show in both PP and XP views. They are typically told apart by using a chemical stain (alizarin red) that turns calcite pink.
- Zircon
- small prismatic crystals with high relief and exteme birefringence, often surrounded by pleochroic halos when present as inclusions in other minerals
- apatite
- colorless, moderate relief, white-to-gray interference colors; often small lathlike prismatic crystlas with a hexagonal cross section; uniaxial
- Monazite
- found in sandstones and siltstones
- Sericite
- .
- Leucoxene
- .
- Hematite
- normally opaque, but thin crystals may show some red color at their edges, red, black or steel blue in reflected light
- Zeolite
- colorless, low to moderate relif, very fine grained. colorless
- Magnetite
- opaque, steel blue-black in thin sections, euhedral crystals are rhombohedral cross sections of octahedra and are magnetic