Sunstone

For other uses, see Sunstone (disambiguation).

Sunstone
General
Category	Crystal
Chemical formula	sodium calcium aluminum silicate (Ca,Na)((AI,Si)2Si2O8)
Identification
Color	clear, yellow, red, green, blue, and copper shiller
Crystal habit	Euhedral Crytals, Granular
Crystal system	Triclinic
Twinning	Lamellar
Cleavage	001 Perfect, 010 Good
Fracture	Conchoidal
Mohs scale hardness	6–7.2
Luster	Vitreous
Streak	white
Diaphaneity	Transparent to Translucent
Density	2.64–2.66
Optical properties	Double Refractive: weak to medium
Refractive index	1.525–1.58
Dispersion	Weak

Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance; this has led to its use as a gemstone. It has been found in Southern Norway, and in some United States localities. It is the official gemstone of Oregon.

Properties

Unpolished sunstone

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. In the middle part of this crystal, it sparks a lot, and usually has a dark color in the middle, and the color becomes lighter as it becomes the outer part.

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."

Distribution

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. Due to the discovery of large deposits in Oregon, Sunstone is now readily available.

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

The "orthoclase sunstone" variant has been found near Crown Point and at several other localities in New York, as also at Glen Riddle in Delaware County, Pennsylvania, and at Amelia Courthouse, Amelia County, Virginia.

Sunstone is also found in Pleistocene basalt flows at Sunstone Knoll in Millard County, Utah.^[1]

Oregon sunstone

Various gem colors of Oregon sunstone

A variety known as Oregon sunstone is found in Harney County, Oregon and in eastern Lake County north of Plush. Only Oregon sunstone contains inclusions of copper crystals.^[2] Oregon sunstone can be found as great as three inches wide. The copper leads to variant color within some stones, where turning one stone will result in manifold hues. The more copper within the stone, the darker the complexion.^[3]

On August 4, 1987, Oregon State Legislature designated Oregon sunstone as its state gemstone by joint resolution.^[4][5]

References

1. ^ Sunstones at Sunstone Knoll, Millard County. Utah Geological Survey, accessed September 14, 2007.
2. ^ McClellan, Jan. Oregon Sunstone, Fact and Fancy. Designer Jewelry.
3. ^ Oregon Sunstone - Official State Gemstone. Nature of the Northwest: Rock Hounding, accessed September 13, 2007.
4. ^ Chapter 186 – State Emblems; State Boundary 2005 Oregon Revised Statutes
5. ^ "Oregon Symbols". SHG Resources. http://www.shgresources.com/or/symbols/gemstone/. Retrieved 2007-09-14. 

 This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 

External links

• Mineralogy Database
• Saab, Patricia. Oregon Sunstone in the Oregon Encyclopedia