HARMOTOME, a mineral of the zeolite group, consisting of hydrous barium and aluminium silicate, Usually a small amount of potassium is present replacing part of the barium. The system of crystallization is monoclinic ; only complex twinned crystals are known. A common and character istic form of twinned crystal consists of four intercrossing indi viduals twinned together according to two twin-laws ; the com pound group resembles a tetragonal crystal with prism and pyra mid, but may be distinguished from this by the grooves along the edges of the pseudo-prism. Crystals are usually white and translucent, with a vitreous lustre. The hardness is 41, and the specific gravity 2.5.
Like other zeolites, harmotome occurs with calcite in the amygdaloidal cavities of volcanic rocks, for example, in the dolerites of Dumbartonshire, and as fine crystals in the agate lined cavities in the melaphyre of Oberstein in Germany. It also occurs in gneiss, and sometimes in metalliferous veins.