Difference between revisions of "Feldspar"
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[[Image:Oligoclase feldspar.jpeg|thumb|250px|left|Oligoclase feldspar from Tanzania.<br />Photo courtesy of Scott
[[Image:Oligoclase feldspar.jpeg|thumb|250px|left|Oligoclase feldspar from Tanzania.<br />Photo courtesy of Scott Davies, american-thai.com]]
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Revision as of 04:32, 2 August 2007
|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 of 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 two known for their phenomenal adularescence and iridescence . Amazonite was used by the ancient Egyptians for carving images of deities, the stone 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 (except for microcline) and have a chemical composition of KAlSi3O8.
- Orthoclase moonstone
- Microcline (amazonite)
Plagioclase feldspars grow in triclinic crystals. Its varieties belong to an isomorphous series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).
- Albite (100-90% albite, 0-10% anorthite)
- Oligoclase (90-70% albite, 10-30% anorthite)
- Andesine (70-50% albite, 30-50% anorthite)
- Labradorite (50-30% albite, 50-70% anorthite)
- Bytownite (30-10% albite, 70-90% anorthite)
- Anorthite (10-0% albite, 90-100% anorthite)