Geoscience Reference
In-Depth Information
Table A7.2
Continued
Mineral
category
Mineral
Features (note - other colours may be possible)
Secondary
Calcite
White or colourless, three cleavages often seen (not at 90°), hardness 3,
fi zzes with dilute HCl
Epidote
(In hydrothermally altered rocks) Apple green, variable but often
elongated crystals. Hardness 6-7
Chlorite
Mid-green to dark greenish-yellow. Often in thin fl akes due to good
cleavage. Hardness 2-3
Zeolites
Usually whitish, fi ne fi brous growths on joint faces or within vesicles
(amygdales). Hardness 5-6
100
muscovite
biotite
amphibole
80
pyroxene
60
Na-rich
plagioclase
feldspar
potassium
feldspar
olivine
40
Ca-rich
20
quartz
0
40
50
60
silica content/wt.% SiO
2
70
rock type
grain size
ultramafic
mafic
intermediate
felsic
fine
<0.25 mm
basalt
andesite
rhyolite
medium
0.25 - 2 mm
dolerite
microdiorite
microgranite
coarse
>2 mm
peridotite
gabbro
diorite
granite
45
52
66
silica content in chemical analysis / wt.% SiO
2
SiO
2
increasing
Fe/Mg increasing
Figure A7.1
Classifi cation scheme for igneous rock types, by grain size and mineralogy. Fine-grained felsic
rocks are paler than fi ne-grained mafi c rocks, and may be reddened by alteration. However,
glassy
rhyolite
(obsidian) can be black.
