Clues for the Identification of Those Pesky Rock Forming Minerals

(Follow along on p. 62)

Colby Geology

A. FELDSPARS (two varieties to identify)All have two directions of cleavage.1. PLAGIOCLASE (2 types to identify) (Ca,Na)(Si,Al)4O8

Have striations on cleavage planes (end at edge of cleavage plane).a. OLIGOCLASE (white) Na-rich feldsparb. LABRADORITE (gray to black) Ca-rich feldspar

LABRADORITEOLIGOCLASE

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) KAlSi308

Has stringers that go through the mineral - looks superficially like striations.

Look at display so that you know what 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 124o and 56o

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

88o and 92o cleavage angles. Crystals usually short and stubby.

a. AUGITE (greenish black)

AUGITE

C. MICAS (3 to identify)All have one direction of cleavage, producing

platy appearance.1. MUSCOVITE (white-silver)2. BIOTITE (brown)3. CHLORITE (green) - may not look like the

other micas in terms of plateyness.

CHLORITE

D. QUARTZ (SiO2)Hardness is 7.Can be many colors due to impurities. Displays conchoidal fracture.

ROSE QUARTZ AMETHYST

CONCHOIDAL FRACTURE

E. OLIVINEOlivine is olive green.Granular appearance.May be able to see conchoidal fracture of grains

with hand lens.

F. CALCITE (CaCO3)Fizzes or effervesces in HCl.Three directions of cleavage not at right angles

(rhombohedral cleavage).Clear pieces may exhibit double refraction.

RHOMBOHEDRALCLEAVAGE HCl ACID REACTION

DOUBLE REFRACTION

G. GARNETReddish-brown color (but can be others).Has hackly fracture, no cleavage.Roundish crystals with 12-24 flat faces

readily visible in most cases.

H. STAUROLITECrystals usually twinned ("fairy crosses").