Difference between revisions of "Feldspar"
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Revision as of 06:08, 19 December 2006
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|--Doos 14:36, 18 December 2006 (PST)|
|Chemical composition|| Potassium, sodium and calcium-aluminum silicates|
I. K-feldspar (KALSi308)
|Crystal system||Monoclinic - triclinic|
|Habit||Prismatic, often twinned|
|Cleavage||Good to perfect|
|Optic nature||Biaxial ±|
|Refractive index|| 1.52 - 1.53 (K-feldspar)|
1.528 - 1.588 (plagioclase)
|Birefringence|| 0.006 - 0.007 (K-feldspar)|
0.008 - 0.011 (plagioclase)
|Specific gravity|| 2.55 - 2,58 (K-feldspar)|
2.60 - 2.80 (plagioclase)
Feldspar is a group of minerals that are very important in rock formation, accounting for over half the earth's crust! There are a number of varieties that are used in jewelry. The most important are andesine, sunstone, amazonite, moonstone, and labradorite, the latter known for their phenomenal iridescence and adularescence. Amazonite was used by the ancient Egyptians for the carving of deities. The stone was considered a catalyst between the living and the gods. Moonstone was thought to drive away sleeplessness.
Feldspars are divided into two types.
- K-feldspars (potassium feldspars).
- Plagioclase feldspars (an isomorphous series between albite and anorthite).
K-feldspars grow in monoclinic crystals and have a chemical composition of KAlSi3O8.
- Orthoclase moonstone
Plagioclase feldspars grow in triclinic crystals and its varieties belong to an isomorphous series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).