GEOS 356 Minerals in Optical Microscopy

Vocabulary flashcards based on lecture notes about minerals under PPL and XPL

Quartz

• A silicate mineral with the formula SiO2

• uniaxial positive

• no color/pleochroism in PPL

• low relief

• 1st order gray to white interference colors in XPL

• often appears anhedral

• exhibits undulose extinction in XPL.

K-feldspar

• KAlSi3O8

• biaxial negative

• no color/pleochroism in PPL

• low relief

• 1st order ~gray interference colors in XPL

• simple (carlsbad) to tartan twinning in microcline

Plagioclase

• NaAlSi3O8-CaAl2Si2O8

• biaxial positive or negative

• no color/pleochroism in PPL

• low relief

• 1st order -gray interference colors in XPL.

• simple (carlsbad) to albite/polysynthetic (piano keys) twinning

• may exhibit chemical zoning (esp in…)

Biotite

• phyllosilicate mineral

• K(Mg,Fe)3AlSi3O10(OH)2

• biaxial negative

• perfect cleavage

• typically brown, brownish green or reddish brown with strong pleochroism

• 2nd-3rd order interference colors (XPL) masked by the body color and mottled "birds-eye" extinction.

Muscovite

• phyllosilicate mineral

• KAl2(AlSi3O10)(OH)2

• biaxial negative

• often tabular

• perfect cleavage

• moderate relief

• colorless in thin section with no pleochroism

• interference colors usually in the second order range, although third order is also common and mottled "birds-eye" extinction.

Hornblende

• amphibole series mineral

• (Ca,Na)2(Mg,Fe+2, Fe+3,Al)5Si6(Si,Al)2O22(OH)2

• biaxial negative

• usually occurs as bladed or prismatic with a diamond shape basal cross-section

• 2 cleavage plane 60/120

• moderate to high relief

• pleochroic in various shades of green and brown

• interference colors 2nd-3rd order, masked by the body colour

• can have simple twinning.

Clinopyroxene (cpx)

• CaMgSi2O6 - CaFeSi2O6

• biaxial positive or negative

• forming stubby prisms elongate along the c-axis, 2 cleavage planes 90 degrees

• high relief

• usually colorless, tan-gray, pale green, pale brown or brownish green in thin section

• weak or nonexistent pleochroism

• interference colors 2nd order

• inclined extinction

• can have simple twinning and exsolution lamellae.

Orthopyroxene (opx)

• Mg2Si2O6 - Fe2Si2O6

• biaxial positive or negative, forming stubby prisms elongate along the c-axis, 2 cleavage planes 90 degrees, high relief, pale pink to green pleochroism, interference colors typically first order greys, high iron may display lower second order, parallel extinction in longitudinal sections and exsolution lamellae.

Garnet

A nesosilicate mineral with the formula (Fe, Mg, Mn, Ca)3Al2Si3O12 and Ca3Fe2Si3O12 (andradite), isometric, rounded grains, massive granular, high relief, pale pink-colourless in PPL, extinct at all angles in XPL and often contains many inclusions.

Olivine

A nesosilicate mineral with the formula Mg2SiO4 - Fe2SiO4, biaxial positive or negative (depending on the Fo content), forming stubby prisms elongate along the c-axis, no cleavage, often fractured, moderate to high relief, often colorless in PPL, high (2nd - 3rd order) interference colors and often altered to serpentine or talc.

Talc

A phyllosilicate mineral with the formula Mg3Si4O10(OH)2, biaxial negative, 1 plane perfect cleavage, moderate relief, fibrous, scaly or dense, often lathy, randomly oriented or subparallel to radiating aggregates, colorless in PPL, third-order interference colors, similar to muscovite, can look similar to micas in cross-polarized light and mottled extinction.

Andalusite (And)

A silicate mineral with the formula Al2SiO5 (low P), moderately high relief, colorless or pale pink, 1st-order interference colors, parallel extinction, 2 cleavages at 90°, can be altered to sericite (fine-grained mica) and can be elongate prism or square "chiastolite" cross-section w/dark inclusions that cross the diagonals.

Kyanite (Ky)

A silicate mineral with the formula Al2SiO5 (hi P), high relief, colorless to darkish in (PPL), 1st-order interference colors (often pale yellow), strong cleavage parallel to long dimension (bladed), prismatic habit, inclined extinction and 2 cleavages at 90°.

Sillimanite (Sil)

A silicate mineral with the formula Al2SiO5 (hi T), moderately high relief, colorless, 2nd-order interference colors, parallel extinction and two forms: Fibrolite aggregates of radiating/swirled/matted fibrous crystals or slender prismatic crystals.

Staurolite (St)

A silicate mineral with the formula MgFeAlSi(OH)x (med-hi P), high relief, yellow pleochroic (PPL), middle 1st-order interference colors, prismatic crystals, commonly "sieved" and famous for its twinned crystals that form into the shape of a cross.

Cordierite (Crd)

A silicate mineral with the formula MgFeAlSi(OH)x (low P), low relief, colorless, 1st-order grey interference color, anhedral form, inclusions common and abundant, lamellar & cyclic twinning masquerading as plag and occurs in medium to high grade metapelites.

Chlorite (Chl)

A silicate mineral with the formula MgFeAlSi(OH)x (low T), low to moderate relief, pleochroic, colorless to pale green, anomalous, low-order interference colors (weird blues & greens, very deep blue), common platy or scaly form similar to micas and low to medium grade metamorphism.

Chloritoid (Cld)

A silicate mineral with the formula MgFeAlSi(OH)x (med-hi P), high relief, variable color: pleochroic green, blue-green, gray-green, colorless, 1st-order yellow interference color masked by its color, tabular form, usually rectangular in cross section, perfect basal cleavage, usually has inclined extinction and low to medium grade metamorphism with "Hourglass" zoning.

Calcite

A carbonate mineral with the formula CaCO3 (low T/P), Colourless, In thin section usually show fine to coarse, anhedral, Perfect rhombohedral cleavage intersect at about 75°, V order interference colors and characterized by change in relief during a 360°rotation of the stage; the crystals is said to "twinkle" during rotation.

Glaucophane

An amphibole mineral with the formula Na2Mg3Al2Si8O22(OH)2, PPL: pleochroic blue to violet, XPL: middle-upper 1st order, masked by mineral color and Amphibole cleavage (60/120), often diamond-shaped (elongate, prismatic sections that look acicular also common)

Riebeckite

An amphibole mineral with the formula Na2Fe2+3Fe3+2Si8O22(OH)2, PPL: strongly colored, inky blue-black to muddy brown showed more dark blue-green, Pleochroism: intense, XPL: Maximum interference colors range to upper first order, but are masked by the strong color and Characteristic amphibole cleavage.

Lawsonite

A silicate mineral with the formula CaAl2Si2O7(OH)2, PPL: usually colorless, XPL: large range from first to second order, 2 perfect cleavages, Often tabular with square cross sections and Can look like cpx.

Omphacite (CPX)

(Ca,Na)(Mg,Fe2+, Fe3+,Al)Si2O6, PPL: colorless to pale green or green and also deep green, XPL: lower 2nd order, Stubby prisms with 4- or 8-sided cross-sections, 2 good cleavages at 90° (px cleavage) and Found in highest-pressure blueschists, defining phase in eclogite (higher P)

Epidote

Ca2Fe3+Al2OSiO11(OH)x, PPL: usually colorless, higher Fe content produces yellow-green pleochroism, XPL: mild to strong birefringence, the colors often vary within the same crystal, upper 1st order to 3rd order (increases with iron content), Commonly forms anhedral grains or granular aggregates with columnar or bladed xl's, High relief and One perfect cleavage

Clinozoisite

Accessory mineral in blueschist and eclogite, PPL: usually colorless, not pleochroic, XPL: 2nd-order purple, High relief, wedge form and absence of green/yellow color and lower birefringence distinguish it from epidote, although clinozoisite is a monoclinic 'epidote group' mineral

Zoisite

Accessory mineral in blueschist and eclogite, Epidote group mineral, High relief, Bladed and acicular, PPL: usually colorless and XPL: Anomalous blue (berlin blue) birefringence sometimes with some yellow in it