# 1911 Encyclopædia Britannica/Vesuvianite

VESUVIANITE, a rock-forming mineral of complex composition. It is a basic calcium and aluminium silicate containing small amounts of iron, magnesium, water, fluorine, etc., and sometimes boron; the approximate formula is H2Ca6(Al,Fe)3Si5O18. It crystallizes in the tetragonal system, but often exhibits optical anomalies, and the optical sign varies from positive to negative. Well-developed crystals are of frequent occurrence. They usually have the form of four- or eight-sided prisms terminated by the basal planes (c) and pyramid-planes (p in fig.); the prism-planes are vertically Striated and the basal planes smooth and bright. Crystals are transparent to translucent, vitreous in lustre and vary in colour from brown to green; a sky-blue variety, called cyprine, owes its colour to the presence of a trace of copper. The specific gravity is 3.4 and the hardness ${\displaystyle \scriptstyle {6{\frac {1}{2}}}}$. The name vesuvianite was given by A. G. Werner in 1795, because fine crystals of the mineral are found at Vesuvius; these are brown in colour and occur in the ejected limestone blocks of Monte Somma. Several other names have been applied to this species, one of which, idocrase of R. J. Haiiy (1796), is now in common use.