Silicates
|
Mafic |
Name
|
Chemical Formula |
|
|
|
Other Diagnostic Properties |
| Olivine |
(Mg,Fe)2SiO4 |
6-7 |
Green, yellow green |
Concodial fracture |
Olivine is a solid solution between Forsterite (Mg2Si4) and Fayalite (Fe2SiO4). Forsterite can be olive-green, light green, dark green, yellow-green, yellow-brown, and brown, rarely white (Alta Stock, Utah Olivines), gray, or orange. Pure Forsterite is colorless, but this is extremely rare. Fayalite is usually yellow-brown to brown. |
| Pyroxene |
Ca(Mg,Fe)Si2O6
(Mg,Fe)2Si2O6 |
5-6 |
Green, gray, white, Bronzite, black |
2 cleavages about 90 degrees |
poor cleavages, short and stubby crystals |
| Amphibole (Hornblende) |
Ca2(Mg,Fe)4Al[AlSi7O22](OH)2 |
5-6 |
Dark green, black |
2 cleavages at about 60/120 |
Elongated crystals, two excellent cleavages that intersect at 120 and 60 degrees (see photomicrograph BH250-42b) |
| Biotite (mica) |
KMg3AlSi3O10(OH)2
KFe3AlSi3O10(OH)2 |
2-2.5 |
Black |
1 perfect direction |
Color is distinctive |
Felsic |
Plagiocalse Feldspar |
NaAlSi3O8
CaAl2Si2O8 |
6 |
White, gray, dark gray, Labradorite |
2 directions at ~90 degrees |
Polysynthetic twinning (1, 2) on best cleavage surface. Closely spaced polysynthetic twinning is often observed as striations |
>
| Alkali Feldspar (Orthoclase) |
KAlSi3O8 |
6 |
White, pink, guff gray |
2 directions at ~90 degrees |
Orthoclase is common mineral, and occurs in numerous environments. It is polymorphous with Microcline and Sanidine. These three minerals form the Alkali Feldspar group (scanned image). They are identical in physical properties, and it is sometimes impossible to distinguish one another without petrographic or x-ray analysis. Microcline crystallizes in the triclinic system, and Orthoclase and Sanidine crystallize in the monoclinic system. Sanidine forms at high temperatures and whereas Orthoclase forms at low temperatures and cools slowly. Pegmaite from Black Hills, South Dakota showing very large orthoclase. |
| Muscovite (mica) |
KAl2AlSi3O10(OH)2 |
2-2.5 |
Colorless |
1 perfect direction |
Color is distinctive. Muscovite from Black Hills, South Dakota |
| Quartz |
SiO2 |
7 |
Colorless, White, Violite Gray |
No cleavage, concoidal fracture |
Glass, hexagonal, prismatic crystal form is distinctive when present. There are many varities of quartz, including; Amethyst, Citrine, Green quartz, Smoky quartz, Rose quartz, Pink quartz, Agate1, 2, Colorless quatrz, Cristobalite |
| |
|
Nonsilicates |
Calcite |
CaCO3 |
3 |
Colorless, white |
3 directions (1, 2) not at 90 degrees |
Rhombic cleavage, reacts (fizzes) with HCl |
| Dolomite |
CaMg(CO3)2 |
3.5 -4 |
White, gray, pink |
3 directions not at 90 degrees |
Reacts (fizzes) less vigorously with HCl (compared to calcite) only on fresh surface or powdered |
| Kaolinite |
Al2Si25(OH)4 |
1.5-2 |
White |
1 direction perfect |
Sticks to tongue, Earthy (dull) luster |
|
