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Feldspar Group

The Feldspars are a group of minerals that have similar characteristics due to a similar structure. All Feldspars have low symmetry, being only monoclinic. They tend to twin easily, occassionally producing parallel layers of twinned crystals. They are relatively hard at around 6, and have an average density of 2.55 to 2.76. They have a rather dull to rarely vitreous luster. Crystals tend to be blocky. Some Feldspars may be triboluminescent. They have two directions of cleavage at nearly right angles. As a matter of fact, the name Feldspar comes from the Greek phrase meaning oblique fracture, because of the angle between the two prominent cleavages.

The Feldspar group of minerals includes these gem types: Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone.

Ther are nearly 20 members recognized in the Feldspar group, but only nine are well known and common. Those few, however, make up the greatest percentage of minerals found in the Earth's crust.

The following are some of the more common Feldspar minerals:

The Plagioclase Feldspars:

Albite (Sodium aluminum silicate)
Oligoclase  (Sodium calcium aluminum silicate)
Andesine  (Sodium calcium aluminum silicate)
Labradorite  (Calcium sodium aluminum silicate)
Bytownite  (Calcium sodium aluminum silicate)
Anorthite  (Calcium aluminum silicate)
 

The Potassium-Feldspars or alkali Feldspars:

Adularia  (Potassium aluminum silicate)
Microcline  (Potassium aluminum silicate)
Sanidine  (Potassium sodium aluminum silicate)
Orthoclase  (Potassium aluminum silicate)
Valencianite  (Potassium aluminum silicate)
 

Feldspar Group Members