Clues for the Identification of Those

1
Clues for the Identification of Those Pesky Rock
Forming Minerals (Follow along on p. 62)
Colby Geology
2
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 feldspar b. LABRADORITE (gray to black)
Ca-rich feldspar
STRIATIONS
LABRADORITE
OLIGOCLASE
3
Labradorite has labradorescence or chatoyance.
LABRADORITE
4
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
5
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
6
b. ACTINOLITE (greenish gray or grayish
green) Crystals usually in radiating masses
(check with me if not visible!)
ACTINOLITE
7
2. PYROXENES (1 to identify) Cleavage
planes intersect forming 88o and 92o cleavage
angles. Crystals usually short and
stubby. a. AUGITE (greenish black)
AUGITE
8
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.
9
(No Transcript)
10
(No Transcript)
11
CHLORITE
12

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

ROSE QUARTZ
AMETHYST
13
CONCHOIDAL FRACTURE
14
E. OLIVINE Olivine is olive green. Granular
appearance. May be able to see conchoidal
fracture of grains with hand lens.
15
F. CALCITE (CaCO3) 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
16
DOUBLE REFRACTION
17
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.
18
H. STAUROLITE Crystals usually twinned ("fairy
crosses").
19
(No Transcript)