141 rock forming minerals

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

OLIGOCLASE

LABRADORITE


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 what striations are

ORTHOCLASE


. FERROMAGNESIANS (iron and magnesium silicate
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,
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 (SiO 2 )
Hardness is 7.
Can be many colors due to
impurities.
Displays conchoidal fracture.

ROSE QUARTZ

AMETHYST

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
CLEAVAGE

HCl ACID REACTION


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").