Silicate of calcium and aluminium.
Crystal system Orti-iorhombic.
Appearance It usually occurs as poorly deﬁned crystals. aggregates of small, possibly rod-shaped crystals. or flattened crystals. Normally white, grayish or greenish, it may also display brilliant colors such as pink, blue and bright green. it has perfect prismatic cleavage.
Physical properties It has a hardness of about 6.5. The density is 3.11-3.40 Q/CITI3 (lower in the massive varieties, higher in distinct crystals}. The refractive indices are about ﬂu 1.69, n-y LTD. Individual. colored crystals may have marked pleochroism.
Genesis The minute crystals distributed in rocks are produced by metamorphism of plagioclases. Larger crystals are sometimes found as well.
Occurrence it is widely distributed wherever metamorphic rocks and gabbro are found together. The varieties used tor gems and ornaments are, however, confined to restricted areas of East Africa (tanzanite and massive green zoisite) and Norway lthulite).