Geology Reference
In-Depth Information
5. Volcanics 2-Pyroclastic Rocks
4. Volcanics 1-Lava Flows
3. Igneous Textures & Classification
2. Field Skills & Outcrop Structures
1. Introduction and Occurrence
Table 3.4
(continued)
Mineral
Typical chemical
formula
Colour
Cleavage
Lustre
Habit
Hardness
Biotite (mica)
K(Mg,Fe)
3
(AlSi
3
O
10
) (OH)
2
Black to dark brown
or green
One excellent
cleavage; cleaves
into thin flexible
sheets
Very shiny
Thin tabular crystals,
occasionally six-sided,
especially in ignimbrites
and acid lavas
2.5-3
Frequent accessory minerals
Tourmaline
Na(Mg,Fe)
3
Al
6
B
3
Si
6
O
22
(OH,F)
4
Black, but varieties
may be blue, red or
green
Very poor
Vitreous
shiny
Long thin prismatic
needle-shaped crystals,
sometimes longitudinally
striated and often in
clusters; occasionally
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