141 rock forming minerals

FELDSPARS two varieties to identifyAll hav e two directions of cleav age.. Hav e striations on cleavage planes end at edge of cleavage plane.. LABRADORITE gray to black Ca-rich feldspar

Clues for the Identification of

Those Pesky Rock Forming Minerals

(Follow along on p 62)

Colby Geology

A FELDSPARS (two varieties to identify)

All hav e two directions of cleav age.

Hav e striations on cleavage planes

(end at edge of cleavage plane).

a OLIGOCLASE (white) Na-rich feldspar

b LABRADORITE (gray to black) Ca-rich feldspar

LABRADORITE OLIGOCLASE

STRIATIONS

Labradorite has labradorescence or chatoyance.

LABRADORITE

A FELDSPARS (two varieties to identify)

All have two directions of cleavage.

2 ORTHOCLASE (pink, tan or white) KAlSi 3 0 8 Has stringers that go through the mineral

- looks superficially like striations.

Look at display so that you know w hat striations are

ORTHOCLASE

B FERROMAGNESIANS (iron and magnesium silicates)

All have two directions of cleavage.

1 AMPHIBOLES (2 types to identify) Cleavage planes intersect forming 124 o and 56 o

cleavage angles.

Crystals usually long and thin.

a HORNBLENDE (black)

Cleavage Angles

b ACTINOLITE (greenish gray or grayish green) Crystals usually in radiating masses

(check with me if not visible!)

ACTINOLITE

2 PYROXENES (1 to identify)

Cleavage planes intersect forming

88 o and 92 o cleavage angles.

Crystals usually short and stubby.

a AUGITE (greenish black)

AUGITE

C MICAS (3 to identify)

All have one direction of cleavage,

1 MUSCOVITE (white-silver)

2 BIOTITE (brown)

3 CHLORITE (green) - may not look like

plateyness.

CHLORITE

D QUARTZ (SiO 2 )

Hardness is 7.

Can be many colors due to

impurities

Displays conchoidal fracture.

CONCHOIDAL FRACTURE

E OLIVINE

Olivine is olive green.

Granular appearance.

May be able to see conchoidal fracture of grains

with hand lens.

F CALCITE (CaCO 3 )

Fizzes or effervesces in HCl.

Three directions of cleavage not at right angles

(rhombohedral cleavage).

Clear pieces may exhibit double refraction

RHOMBOHEDRAL

DOUBLE REFRACTION

G GARNET

Reddish-brown color (but can be others) Has hackly fracture, no cleavage.

Roundish crystals with 12-24 flat faces

readily visible in most cases

H STAUROLITE

Crystals usually twinned ("fairy crosses")