1911 Encyclopædia Britannica/Sylvite
SYLVITE, a mineral consisting of potassium chloride (KCl), 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 cubo-octahedra 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 1.98. Like salt, it is highly diathermanous. The name sylvite or sylvine is from the old pharmaceutical name, sal digestivus sylvii, for this salt.