Geoscience Reference
In-Depth Information
Appendix A9: Metamorphic
Table A9.1 Properties useful for fi eld identifi cation of some common metamorphic minerals. Mohs' scale of
hardness (H) is given in Appendix A1, Table A1.1.
Mineral (abbreviation)
formula
Colour; lustre
Typical habit
Cleavage;
twinning
Hardness (H)
Quartz (Qtz)
SiO 2
Grey, greasy or
glassy; white in
veins
Interstitial,
irregular grains
None
7
K-feldspar (Kfs)
KA1Si 3 O 8
Pink, white or grey;
glassy, but chalky
when weathered
Stumpy, tabular
crystals
2 cleavages;
simple twins
common
6
Plagioclase (Pl)
NaA1Si 3 O 8 -CaA1 2 Si 2 O 8
White or grey;
glassy or pearly
Tabular crystals or
ovoid grains
2 cleavages;
multiple or
simple twins
6-6.5
Chlorite (Chl)
(MgFe) 5 A1 2 Si 3 O 10 (OH) 8
Dark 'bottle' green;
pearly
Flexible fl akes; fi ne
groundmass
1 perfect
cleavage
2-3
Biotite (Bt)
K(Mg,Fe) 3 A1Si 3 O 10 (OH) 2
Dark brown to
black; pearly
Elastic fl akes;
forms foliations
1 perfect
cleavage
2.5-3
Muscovite (Ms)
KA1 3 Si 3 O 10 (OH) 2
Silvery-white to pale
golden; pearly
Elastic fl akes;
forms foliations
1 perfect
cleavage
2-2.5
Andalusite (And)
A1 2 SiO 5
White, grey, dull
red; glassy
Euhedral prisms;
square
cross-sections
2 cleavages
6.5-7
Kyanite (Ky)
A1 2 SiO 5
Pale blue to white,
grey; pearly to
glassy
Bladed crystals;
porphyroblasts
2 cleavages
4.5-7
Sillimanite (Sil)
A1 2 SiO 5
White, grey;
glassy, or pearly
when fi brous
Slender prisms, or
fi brous; forms
foliations
1 cleavage
7
Garnet (Grt)
(Mg,Fe,Ca,Mn) 3 A1 2 Si 3 O 12
Red to orange,
pink, violet, brown,
black, green; glassy
Equant crystals,
dodecahedral,
robust
None
6.5-7.5
Staurolite (St)
(Mg,Fe) 4 A1 18 Si 7.5 O 44 (OH) 4
Stumpy, tabular
crystals;
porphyroblasts
1 cleavage;
twinning
common
7-7.5
Brown, orange,
yellow, black;
glassy, resinous or
dull
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