WGNHS
Wisconsin Geological and Natural History Survey

Anorthoclase

Anorthoclase as “moonstone” with characteristic schiller. Sample is about 20 cm across. UW–Madison Geology Museum sample. (Photo by Pete Rodewald.)

Formula: (Na,K)AlSi3O8 Triclinic

Description:

An unpolished anorthoclase chunk which is light brown, gray, and blue in color.
Anorthoclase (unpolished) showing “moonstone” schiller. Stettin, Marathon County County, Wisconsin. (Photo from Bill Schoenfuss, Moonlight Photography.)

Anorthoclase is a high-temperature solid solution series found in certain volcanic and plutonic rocks.

Polished anorthoclase with blue "moonstone" schiller on the left side of the mineral fragment.
Polished anorthoclase showing “moonstone” schiller. Stettin, Marathon County County, Wisconsin. (Photo from Bill Schoenfuss, Moonlight Photography.)

MARATHON COUNTY: Coarse microperthitic anorthoclase showing a schiller “moonstone” occurs in a quarry in pyroxene syenite in the Stettin pluton just north of Stettin Road in the SE sec. 14 T.29N. R.6E. Xls. up to 35 cm. long occur in this pluton. At last report, the quarry is water-filled, hazardous and posted “No Trespassing”. It should be avoided unless permission is granted by the owner (LaBerge and Myers, 1983; Myers et al., 1984; Falster et al., 2000).

Polished anorthoclase showing a silver and blue colored “moonstone” schiller.
Polished anorthoclase showing “moonstone” schiller. Stettin, Marathon County County, Wisconsin. (Photo from Bill Schoenfuss, Moonlight Photography.)