Orthoclase

Orthoclase from Flinders Island, Tasmania, Australia, measuring 120 × 60 × 55 mm and weighing 695 grams. That is a substantial, solid piece of feldspar, comfortably filling the hand, and it comes from one of the more interesting granite islands in Bass Strait.

Orthoclase is the monoclinic potassium feldspar, KAlSi3O8, and together with Quartz and the plagioclase feldspars it makes up the bulk of most granites and the coarse pegmatites that grow from them. It is the reference mineral for hardness 6 on the Mohs scale, it breaks along two good cleavages at close to right angles, and it is well known for twinning, the Carlsbad twin being the classic example. Flinders Island is built largely from Devonian granite, and the cavities and pegmatitic patches in that granite are what give the island its reputation among collectors.

Most people know Flinders Island for its Topaz, the water-clear "Killiecrankie diamonds" washed out of the granite country around Killiecrankie Bay, and for Smoky Quartz from the same rocks. The feldspar that hosts all of that is far less often offered, and this Orthoclase, from the Jon Mommers Collection, is a good-sized way to show the granite side of the island's story.