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Amazonite -

COMPOSITION:
The word feldspar literally translates to: feld, meaning field, and spar, meaning "easily cleaved material". It refers to any of several crystalline aluminosilicate minerals found in abundance in the earth's crust. This group of minerals might, with some justification, be regarded as varieties of a single mineral species. With this premise in mind. Feldspar, rather than quartz, could be considered the most abundant of all minerals. All are aluminum silicates of soda, potash, or lime (with a few rarer varieties), and all are closely related in structure and composition. The imp- ortance of the group lies in the fact that feldspars are the principal constituents of igneous and plutonic rocks. There have been over 40 feldspars identified. It should be noted that many of the feldspars are polymorphs, meaning they have the same chemistry. But different structures and therefore are different minerals. The feldspars in the lists below are some of the more commonly collected feldspars.

The Plagioclase Feldspars:
Albite, (Sodium Aluminum Silicate)
Andesine, (Sodium calcium aluminum silicate)
Anorthite, (Calcium aluminum silicate)
Labradorite, (Calcium sodium aluminum silicate)
Bytownite, (Calcium sodium aluminum silicate)
Oligoclase, (Sodium calcium aluminum silicate)

The K-feldspars or Alkali Feldspars:
Microcline, (Potassium aluminum silicate) (Information)
Orthoclase, (Potassium aluminum silicate)
Sanidine, (Potassium sodium aluminum silicate)

DISTINGUISHING CHARACTERISTICS:
Often, feldspars are simply referred to as plagioclase and orthoclase, because identification to greater precision is difficult with ordinary methods. The list below is a list of feldspars commonly used as gemstones and therefore easy to distinguish.

Labradorite - semitranslucent gray that displays a broad iridescent color effect.
Moonstone - semitransparent colorless with white to pale blue floating light when turned. Sunstone - translucent to semitranslucent resembles moonstone, body color is yellowish-orange, billowy golden sheen as the stone is turned
Cat's Eye (Moonstone) - semitransparent strong colors of green, light to dark brown almost black.
Amazonite - opaque greenish blue verity of microcline feldspar.

ENVIRONMENT:
Feldspars are the most widespread of any mineral group and constitute 60% of the Earth's crust; they occur as components of all kinds of rocks (crystalline schists, migmatites, gneisses, granites, most magmatic rocks) and as fissure minerals in clefts and druse minerals in cavities. Feldspars are usually white or nearly white and clear and translucent. They have no color of their own but are frequently colored by impurities

CRYSTAL DESCRIPTION:
Because feldspars have low symmetry, crystals tend to be blocky, being only monoclinic,
2/m, to triclinic, bar 1. They tend to twin easily and one crystal can even be multiply twinned
on the same plane, producing parallel layers of twinned crystals. They have two directions of cleavage at nearly right angles. Feldspars tend to crystallize in igneous environments, but are also present in many metamorphic rocks.

We haven't been able to locate a list of all of the feldspars but here is what we have so far.
ALBITE: NaAlSi3O8
ANDESINE: (Na,Ca)(Si,Al)4O8
AMAZONITE KAlSi3 O8
ANORTHITE: CaAl2Si2O8
ANORTHOCLASE: (Na,K)AlSi3O8
BANALSITE: BaNa2Al4Si4O16
BUDDINGTONITE: (NH4)AlSi3O8,5H2O
BYTOWNITE: (Ca,Na)(Si,Al)4O8
CELSIAN: BaAl2Si2O8
DMISTEINBERGITE: CaAl2Si2O8
HYALOPHANE: (K,Ba)Al(Si,Al)3O8
LABRADORITE: (Ca,Na)(Si,Al)4O8
MICROCLINE: KAlSi3O8
OLIGOCLASE: (Na,Ca)(Si,Al)4O8
ORTHOCLASE: KAlSi3O8
PARACELSIAN: BaAl2Si2O8
REEDMERGNERITE: NaBSi3O8
RUBICLINE: (Rb,K)AlSi3O8
SANIDINE: (K,Na)(Si,Al)4O8
SLAWSONITE: (Sr,Ca)Al2Si2O8
STRONALSITE: SrNa2Al4Si4O16
SVYATOSLAVITE: CaAl2Si2O8

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