Albite may occur in simple crystals, in which case the two perfect cleavage planes meet at an angle of 86° 24′. However, it is much more frequently found twinned in the multiple manner, the individual crystals often being as thin as paper. This gives rise to a fine striation on the end of a crystal, or on the surface made by the imperfect cleavage plane. Where the crystals are extremely thin, the surface may have a pearly luster. Albite types of granite often inclose secondary minerals, that are prized as gems, such as topaz, tourmaline, and beryl.
It is found at Paris, Me., Chesterfield, Mass., Acworth, N. H., Essex Co., N. Y., Unionville, Penn., and in Virginia, and throughout the Rocky Mountains.
[Oligoclase]
(NaCa)AlSi₃O₈
Generally found in cleavable masses in granites and lavas, rarely in crystals; hardness, 6; specific gravity, 2.65; color white, greenish or pink; luster vitreous; translucent on thin edges.
Oligoclase is a plagioclase feldspar and is distinguished by its two perfect cleavage planes meeting at an angle of 86° 32′, but otherwise it is very like albite. Crystals are not common, and it occurs mostly in masses, making one of the components of granite or lava.
It is found in St. Lawrence Co., N. Y., Danbury and Haddam, Conn., Chester, Mass., Unionville, Penn., Bakersville, N. C., etc.
[Labradorite]
(NaCa)AlSi₃O₈
[Pl. 35]
Usually found in cleavable masses in granites and lavas; hardness, 6; specific gravity, 2.71; color gray or white, often with a play of colors; luster vitreous; translucent on thin edges.
Labradorite is distinguished by having the two perfect cleavage planes meet at 86° 14′. The iridescent play of color is also very characteristic and is generally present. It is due to the inclusion of minute impurities. This feldspar is usually associated with granites or lavas in which the dark minerals predominate. It gets its name from being the feldspar of the granites of Labrador, and is also found in the granites of the central part of the Adirondack Mountains and the Wichita Mountains of Arkansas.