Oligoclase

Oligoclase is a sodium-rich member of the plagioclase feldspar series, intermediate between albite and andesine. It is a rock-forming mineral of granites, syenites and gneisses, and it includes two well-known gem and ornamental varieties: aventurescent sunstone and some moonstone.

Properties

Plagioclase is triclinic with two good cleavages meeting at close to 90 degrees and frequently shows fine repeated (polysynthetic) twinning as parallel striations on cleavage faces. Oligoclase is hard (6–6.5) and usually colourless, white or pale grey; sunstone owes its spangled glitter to tiny oriented platelets of hematite or copper.

Occurrence

It is widespread in intermediate igneous and metamorphic rocks. Gem sunstone oligoclase comes from Norway, India and the United States. It sits within the feldspar group alongside the other plagioclase members.