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Feldspar
Feldspar is a group of minerals which are important in the formation of many rocks. There are a few gem varieties of feldspar, the most important of which is moonstone.
All feldspars are types of aluminium silicate, with potassium, calcium, sodium or barium.

Feldspar Types

Colour:
The most desired colour of moonstone is blue, but it also occurs in grey, pink, green and brown. Amazonite is green, labradorite is usually blue. Sunstone, sometimes called aventurine feldspar, is yellow to red. There is also a yellow form of orthoclase gemstone, which owes its colour to the presence of iron.

More About Plagioclase
We have stated above that plagioclase is an isomorphous series of albite and anorthite. Mineralogists have given names to minerals falling within certain bands. The following table gives these names and shows the percentages of albite and anorthite which constitute each variety.
Plagioclase Varieties
VarietyAlbite %Anorthite %
Albite90 - 1000 - 10
Oligoclase70 - 9010 - 30
Andesine50 - 7030 - 50
Labradorite30 - 5050 - 70
Bytownite10 - 3070 - 90
Anorthite0 - 1090 - 100

Characteristics
Chemical Composition and name - Orthoclase & MicroclineKAlSi3O8 - Potassium Aluminium Silicate
Chemical Composition and name - AlbiteNaAlSi3O8 - Sodium Aluminium Silicate
Chemical Composition and name - AnorthiteCaAl2Si2O8 - Calcium Aluminium Silicate
Chemical Composition and name - CelsianBaAl2Si2O8 - Barium Aluminium Silicate
Hardness6 - 6.5
Specific Gravity - Orthoclase2.55 - 2.58
Specific Gravity - Microcline2.54 - 2.57
Specific Gravity - Oligoclase2.62 - 2.65
Specific Gravity - Labradorite2.70 - 2.72
Crystalline System - OrthoclaseMonoclinic
Crystalline System - MicroclineTriclinic
Crystalline System - OligoclaseTriclinic
Crystalline System - LabradoriteTriclinic
Refractive Index - Orthoclase1.52 - 1.53 to 1.53 - 1.54
Refractive Index - Microcline1.52 - 1.53
Refractive Index - Oligoclase1.54 - 1.55
Refractive Index - Labradorite1.56 - 1.57
Bi-refringence - Orthoclase & Microcline0.008
Bi-refringence - Sunstone & Labradorite0.001
Optic Sign - Orthoclase, Sunstone, MicroclineNegative
Optic Sign - LabradoritePositive
Optical CharacterBiaxial
Dispersion0.012

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