VESUVIANITE  , a

rock-forming mineral of complex com- position . It is a basic calcium and aluminium silicate con- aining small amounts of iron, magnesium, water, fluorine, etc., and sometimes boron; the ap- proximate formula is H2Ca6(Al,Fe)aSi5O18 . It crystallizes in the tetragonal system, but often exhibits optical anomalies, and the optical sign varies from positive to negative . Well-devel- oped 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 62 . 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 .

Hauy (1796), is now in common use . Vesuvianite is typically a mineral of contact-metamorphic origin, occurring most frequently in crystalline limestones at their contact with igneous rock-masses; it also occurs in serpentine, chlorite-schist and gneiss, and is usually associated with garnet, diopside, wollastonite, &c . Localities which have yielded fine crystallized specimens are the Ala valley near Turin, Piedmont, Monte Somma (Vesuvius), Monzoni in the Fassa valley, Tirol, Achmatovsk near Zlatonst in the Urals, the River Wilui district near Lake Baikal in Siberia (" wiluite "), Christiansand in Norway, &c . When found in transparent crystals of a good green or brown colour it is occasionally cut as a gem-stone . A compact variety, closely resembling jade in appearance, has been used as an ornamental stone . (L . J .