SYLVITE  , a

mineral consisting of potassium chloride (KCI), first observed in 1823, as an encrustation on Vesuvian lava . Well-formed crystals were subsequently found in the salt deposits of Stassfurt in Prussia and Kalusz in Austrian Galicia . It crystallizes in the cubic system with the form of 'cubes and cubooctahedra and possesses perfect cleavages parallel to the faces of the cube . Although the crystals are very similar in appearance to crystals of common salt, they are proved by etching experiments to possess a different degree of symmetry, namely plagihedral-cubic, there being no planes of symmetry but the full number of axes of symmetry . Crystals are colourless (sometimes bright blue) and transparent; the hardness is 2 and the, specific gravity 1'93 . Like salt, it is highly diathermanous . The name sylvite or sylvine is from the old pharmaceutical name, sal digestivus sylvii, for this salt . (L . J .