Sunstone

Sunstone
General
Category	Mineral variety
Formula (repeating unit)	(Ca,Na)((Al,Si)2Si2O8)
Crystal system	Triclinic
Identification
Color	Colorless, yellow, red, green, blue, and copper shiller
Crystal habit	Euhedral crystals, granular
Twinning	Lamellar
Cleavage	001
Diaphaneity	Transparent to translucent
Specific gravity	2.64–2.66
Refractive index	1.525–1.58

Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a spangled appearance. It has been found in Southern Norway, Sweden and in various United States localities.

The optical effect appears to be due to reflections from inclusions of red copper, in the form of minute scales, which are hexagonal, rhombic, or irregular in shape, and are disposed parallel to the principal cleavage-plane. These inclusions give the stone an appearance something like that of aventurine, hence sunstone is known also as "aventurine-feldspar." The optical effect called shiller and the color in Oregon Sunstone is due to copper. The middle part of this crystal sparkles, and usually the color is darkest in the middle and becomes lighter toward the outer edges.

The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone."

Sunstone was not common until recently. Previously the best-known locality being Tvedestrand, near Arendal, in south Norway, where masses of the sunstone occur embedded in a vein of quartz running through gneiss.

Other locations include near Lake Baikal in Siberia, and several United States localities—notably at Middletown Township, Delaware County, Pennsylvania; Plush, Oregon; and Statesville, North Carolina.

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