About Feldspar I

1.      What is Feldspar?

The feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8 – BaAl2Si2O8) are a group of frame-like structure anhydrous aluminosilicate minerals, which are the most common mineral in the Earth, making up as much as 60% of the Earth's crust.

Generally speaking, there are three categories of feldspar: orthoclase (K-feldspar), plagioclase and celsian. Feldspar has two directions of cleavage that meet at close to a 90 degree angle. The included angle between two sets of cleavages of orthoclase is equal to 90 degrees, and less than 90 degrees for plagioclase.

Orthoclase (Or)

Orthoclase is a blanket term of orthose, sanidine and microcline. In theory, it is composed of SiO2 64.7%, Al2O3 18.4% and K2O 16.9%.

Crystalline System: orthose and sanidine are monoclinic system, microcline is triclinic system.

Crystal: stumpy (short column)

Aggregate: granulose or block-shaped

Hardness: 6-6.5               

Density: 2.57 g/cm3

Melting Point: 1200℃, decomposed to leucite at 1170℃ and liquated at 1686℃

Color: fresh red, gray, grayish white, etc.

Plagioclase

The Plagioclase series is a group of related feldspar minerals, including albite, oligoclase, andesine, laboratories, bytownite and anorthite, that essentially have the same formula but vary in their percentage of sodium and calcium.

Na [Al Si 3O8] （Ab）——Ca [Al2 Si 2O8] （An）
Albite钠长石	An0-10	Ab90-100
Oligoclase奥长石	An10-30	Ab70-90
Andesine中长石	An30-50	Ab50-70
Labradorite 拉长石	An50-70	Ab30-50
Bytownite  培长石	An70-90	Ab10-30
Anorthite 钙长石	An90-100	An0-10

Feldspar Table

Albite is composed of SiO2 68.7%, Al2O3 19.5% and K2O 11.8%.

Crystalline System: triclinic system

Crystal: block or flat column shaped

Aggregate: granulose or block-shaped

Hardness: 6-6.5               

Density: 2.61 g/cm3

Melting Point: 1180℃

Color: white-gray, greenish, fresh red, etc.