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36 Cards in this Set

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*scotch plaid
*lamellar intergrowth
*one/two cleavages
-2V = 77 - 84
CaAl2Si2O8 - NaAlSi3O8
*polysynthetic twinning
*clear color
-white to pale yellow interference colors
*lack of cleavage/alteration/twinning
*undulatory extinction!
-could show fracture
-very low relief
- (+)
*hexagonal basal sections
*often contains fluid inclusions
-colorless, maybe pale yellow/green
-moderate relief
*not perfect twins
*pseudohexagonal basal sections
- (-)
*high interference colors
*platy masses
*fibrous aggregates
-talc + quartz = RARE!
-could be pryophyllite
-one perfect cleavage
*brown to yellow,red or green mica
*one excellent cleavage
*shows bird's eye extinction
-parallel extinction
*clear mica
*2nd order interference colors
*bird's eye extinction
-pale green ? slightly pleochroic
-one excellent cleavage
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.
Chlorite is common as a secondary mineral, forming after mafic minerals, in rocks of many types. It may also be a primary mineral in low- to medium-grade metamorphic rocks.
*dark brown
*pale yellow
* or green mica- like
*strong pleochroism
*shows 2 cleavages
low-grade regional metamorhpic rocks
-associated with chlroite, muscovite, garnet, actinolite, calcite or epidote
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.
Chloritoid is found in low- to meidum-grade metasedimentary rocks. It has general compostion (Fe,Mg,Mn)Al2SiO5(OH)2. It is a layered silicated, similar to chorite and to micas in some ways.
Keys to identification are staurolite's yellow color and pleochrosim, and its occurrence in medium grade pelitic rocks.
·Appearance and habit - Staurolite has diagnostic pleochroic yellow color. It often contains many quartz inclusions, sometimes giving a "swiss cheese" appearance.
Keys to identification are high relief, bladed habit, good cleavage and first order interference colors.
·Color - Typically colorless but sometimes has a pale blue color. May be slightly pleochroic.
Cordierite can be difficult to identify if untwinned, inclusion free, and unaltered. It can be clear with birefringence equivalent to quartz and plagioclase. When twinned, the twins may be distinctive truncating polysyntheitc twins, but may also appear very much like typical plagioclase twins. Cordierite often alters partially or whooly to a fine grained sericite, termed pinite. Inclusions of zircons may give rise to burn marks around included grains, appearing sometimes as brown rings or pleochroic halos.
·Twinning common - Polysynthetic twins may look like plagioclase but frequently pinch out in one direction.
Keys to identification are high relief, low birefringence and parallel extinction.
·Appearance and habit - often euhedral crystals with square outlines or coarse columnar aggergates
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.
·Appearance and habit - Classic sillimanite forms needles with square cross sections that show one diagonal cleavage. It may also appear as blades or as fine fibrous mats (called fibrolite).
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.
·Appearance and habit - Euhedral crystals may have 6 or 8 sides. More typically garnet is subhedral equant or anhedral.
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).
·Color - Hornblende is generally pleochroic in shades of brown and green, less commonly yellow or red-brown. Concentric or patchy color zonation may be present. Actinolite is similar but colors are restricted to greens.
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.
·Extinction - Extinction is parallel in elongate grains (not in grains in other orientations).
Tremolite is identified by its bladed habit, amphibole cleavage, clear to pale green color and upper-first to low-second order interference colors.
·Color - Tremolite is clear to light green, may be slightly pleochroic. The color and pleochroism are more pronounced with greater Fe content.
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.
Olivine is typically found in mafic to ultramafic igneous rocks and in their metamorphic equivalents. Less commonly it is found in marbles and a few other metamorphic rock types. Olivine has general formula (Mg,Fe)2SiO4. Most natural olivines contain only minor other components.
·Appearance and habit - Spinels are isotropic, have high relief, and show no cleavage. They may form octahedra but, more commonly, are anhedral.
·Color - Color is highly variable. Common spinels may be colorless, green, red, brown, blue, or black.
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.
·Color - Common augite is generally light green; Fe-free clinopyroxene such as diopside may be clear; other varieties may be light brown or yellow. Darker color and weak pleochroism are characteristic of more Fe-rich varieties.
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.
Well developed end sections may appear as blocky four- or eight-sided crystals; more typically orthopyroxene forms stubby prisms, rectangular lathes, or anhedral crystals and masses. Basal sections show cleavage angle near 90o. Longitudinal sections show one cleavage. Extinction is parallel to the cleavage.
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.
·Appearance and habit - The characteristic euhedral crystals, if present, are shaped like diamonds, wedges, or distorted hexagons. Typically appears as small accessory grains that stand out from other minerals.
Keys to identifying rutile are its strong yellowish to reddish brown color, high relief, and extreme birefringence.
Rutile is a common accessory mineral in intermediate to mafic igneous rocks, and in many metamorphic rocks. It has composition TiO2.
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.
Graphite is a native element, having composition very close to 100% carbon. It is a widespread accessory mineral in schists, marbles, gneisses, and other metamorphic rocks. It is also, much less commonly, found in some igneous rocks.
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.
·Twinning common - depending on grain orientation, one or two sets of parallel twins may show. The carbonates are the only common minerals that typically show twinning in both PP and XP views.
small prismatic crystals with high relief and exteme birefringence, often surrounded by pleochroic halos when present as inclusions in other minerals
tetragonal, crystlas may appear as small prisms with pyramidal terminations or small slender rounded grains
colorless, moderate relief, white-to-gray interference colors; often small lathlike prismatic crystlas with a hexagonal cross section; uniaxial
hexagonal, sometimes course anhedral crystals, more typicall small subhedral to euhedral hexagonal crystals or prisms
found in sandstones and siltstones
twinning common
normally opaque, but thin crystals may show some red color at their edges, red, black or steel blue in reflected light
very high relief, common imineral in many rocks
colorless, low to moderate relif, very fine grained. colorless
heulandite, stilbite, natrolite, chabazite
opaque, steel blue-black in thin sections, euhedral crystals are rhombohedral cross sections of octahedra and are magnetic
massive or granular aggregates are also common, contact or lamellar twis are commone