Home >> Encyclopedia-britannica-volume-01-a-anno >> Almendralejo to Altwasser >> Alstonite

Alstonite

Loading


ALSTONITE, a member of the aragonite group of minerals. It consists of an isomorphous mixture of calcium and barium carbonates in various proportions and thus differs chemically from barytocalcite (q.v.) which is a double salt of these carbonates in equal molecular proportions. Being isomor phous with aragonite, it crystallizes in the orthorhombic system, but simple crystals are not known. The crystals are invariably complex twins and have the form of doubly terminated pseudo hexagonal pyramids, like those of witherite but more acute. The crystals are translucent and white, sometimes with a shade of pink. Sp. gr. 3. 706 ; hardness 4-44. The mineral has been found at only three localities, all of which are in the north of England. It takes its name from Alston in Cumberland, where it occurs in veins of lead ore.

crystals