Laboratory Manual for Introductory Geology 4e

APPENDIX 3.2
Determinative Tables for Systematic Mineral Identification
Sequence of questions: Luster? Approximate hardness? Streak? Breakage? Color? Other?
TABLE 1 Minerals with metallic luster
(a) Hardness less than 2.5 (softer than a fingernail)
Streak
Cleavage or
fracture
H Color Other diagnostic
properties
Mineral name
(composition)
Black
Perfect cleavage
in one direction
1 Dark gray-black Greasy feel; leaves a mark on
paper; SpG = 2.23
Graphite
C
Yellowbrown
Difficult to see — Yellow-brown Very rarely in masses
with metallic luster;
more commonly dull, earthy;
SpG = 3.6–4
Limonite
FeO(OH) ⋅ nH 2
O
(b) Hardness between 2.5 and 5.5 (harder than a fingernail; softer than glass)
Streak
Cleavage or
fracture
H Color Other diagnostic
properties
Mineral name
(composition)
Gray
Three directions
at 90° angles
2.5 Lead gray Commonly in cubic crystals;
SpG = 7.4–7.6
Galena
PbS
Rarely seen 3 Bronze-brown
when fresh
Commonly with purplish, iridescent
tarnish; SpG = 5.06–5.08
Bornite
Cu 5
FeS 4
Black
Rarely seen 3.5–4 Brassy yellow Often tarnished; similar to pyrite
but not in cubes
Chalcopyrite
CuFeS 2
Rarely seen 4 Brown-bronze Slightly magnetic; SpG = 4.62 Pyrrhotite
Fe 1–x
S
Copper-red
Rarely seen 2.5–3 Copper Often in branching masses;
SpG = 8.9
Copper
(Cu)
(c) Hardness greater than 5.5 (harder than glass; cannot be scratched by a knife)
Streak
Cleavage or
fracture
H Color Other diagnostic
properties
Mineral name
(composition)
Conchoidal
fracture
6–6.5 Brassy yellow Commonly in 12-sided crystals or
cubes with striated faces;
SpG = 5.02
Pyrite
FeS 2
Black
Rarely seen 6 Iron black Strongly magnetic; SpG = 5.18 Magnetite
Fe 3
O 4
Reddish
brown
Rarely seen 5.5–6 Silver white Streak smells like garlic because of
arsenic; SpG = 6.07
Rarely seen 5.5–6.5 Black, red Black variety is metallic; red
variety is more common and has
nonmetallic, earthy luster
Arsenopyrite
FeAsS
Hematite
Fe 2
O 3
78 CHAPTER 3 MINERALS