Geology Reference
In-Depth Information
Muscovite
(mica)
KAl
2
(AlSi
3
O
10
)
(OH)
2
Colourless to pale
brown or green
One excellent
cleavage, cleaves
into thin flexible
sheets
Shiny, silver
and pearly
Tabular crystals sometimes
6-sided, especially in
pegmatites
2-2.5
Mafic minerals
Olivine
(Mg,Fe)
2
SiO
4
Olive green,
yellow-green,
sometimes brown
Very poor,
usually fractures
Glassy when
fresh,
vitreous
when altered
Usually rounded anhedral
crystals, occasionally
equidimensional tabular
forms
6-7
Pyroxene
(i) (Mg, Fe,
Ca)
2
Si
2
O
6
(augite,
etc.)
Black to dark green
or brown
Yellowish-green
Two good sets
meeting at
87
◦
/93
◦
Vitreous
when fresh,
dull when
altered
Four- or eight-sided
prismatic crystals
occasionally showing
cleavage or Aegirine more
acicular
6
(ii) NaFeSi
2
O
6
(aegirine)
