Geology Paper Sample on Basic Mineral Properties

For the first part please answer the above the questions for the following sets of minerals under thin section.

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Set 1

Garnet

It is nonopaque

Color: Colorless to pinkish

Apparent relief: Very high +

Crystal shape: Rounded or equant

Crystal habit: If well-formed it could have 6 or 8 sides in thin section

Cleavage:

Alteration produced: May have trails or sections rich in inclusions of quartz, biotite

Under XPL

It is isotrophic

Range of interference: Nil

No twinning

Set2

Quartz, Plagioclase, K-feldspar

They are nonopaqueHave no pleochroism. Are colorless in thin section.

Crystal shape: Triclinic

Crystal habit: Anhedral or euhedral grains

Apparent relief:

Alpha = 1.527-1.577

Beta = 1.531-1.585

Gamma = 1.534-1.590

Cleavage: (110) is extremely poor cleavage, (010) quite good cleavage, and (001) is an impeccable cleavage. Cleavages intersect at approximately 93-94 degrees.

Alteration produced: No alteration is produced.

Under XPL

It is anisotrophic

Range of interference colors: 0.007-0.013

Maximum birefringence colors under thin section are normally white or first-order grey.

Maximum extinction angle inclined

No twinning.

Set3

Olivine, Orthopyroxene, Clinopyroxene, Biotite, Hornblende

It is opaque.

It is pleochroic. Normally colorless, gray, pale brown or pale green, darker colors linked to iron-rich varieties. Pale pink to green pleochroism.

Crystal shape: Orthorhombic

Crystal habit: Irregular, stubby prismatic, and massive. Longitudinal areas typically rectangular.

Apparent relief:

High-positivenalpha=1.664-1.745nbeta=1.672-1.753ngamma = 1.694-1.771

Cleavage: Archetypal pyroxene cleavages parallel to {110} that intersect at right angle.

Alteration produced: In most cases, it changes to a fg, light colored amphibole (uralite) or may alter to chlorite, serpentine, biotite, carbonates as well or other silicates.

Under XPL

It is anisotrophic.

Range of interference colors

Lower to mid second order interference.

Maximum birefringence of 0.018-0.034

Maximum extinction angle of 480

It is twinned, simple in addition to lamellar twins and composition planes that may blend to create a herringbone pattern.

After answering the questions, please summarize the characteristics features for identification of each of the above minerals under a petrographic microscope. (Garnet, Olivine, Orthopyroxene, Clinopyroxene, Amphibole, Biotite, Plagioclase, K-spar, Quartz)

Garnet

High relief in thin section. In thin section, the color is a pale form of the hand sample color; in addition, it is normally more lightly colored compared to spinel.

Olivine

Rounded or equant crystals with curving cracks in addition to colorless serpentine. Lacks color and cleavage and has a distinctive birefringence.

Orthopyroxene

First-order colors, low birefringence. Thin wavy and irregular lamellae. Parallel extinction in longitudinal zones, pale pink-green pleochroism.

ClinopyroxeneInclined extinction, second-order interference colors, higher birefringence, optimally positive than opx, and low 2V.

Amphibole

Has intense inky blue colors entirely distinctive. Normally have poikilitic plates instead of euhedral xls.

Biotite

Perfect mica cleavage in many areas. Palest when cleavage E-W. Basal areas have low birefringence, no cleavage, as well as little or no pleochroism.

Plagioclase

Cleavage and hardness (6-6.5). Lacks twinning and pleochroism. Chemical zoning is apparent; the calcic center has a bigger extinction compared to the sodic rim.

K-spar

Normally portrays good cleavage and possesses a dusty appearance from minute alteration inclusions. The inclusions comprises of sericite and muscovite, and indicate high interference colors.

Quartz

Have irregular grains, unaltered and clear. Could also indicate strained extinction.