BORACITE, a mineral of special interest on account of its optical anomalies. Small crystals bounded on all sides by sharply defined faces are found in considerable numbers embedded in gypsum and anhydrite in the salt deposits at Luneburg, in Han over, where it was first observed in 178 7. In external form these crystals are cubic with inclined hemihedrism, the symmetry being the same as in blende and tetrahedrite. Their habit varies accord ing to whether the tetrahedron, the cube or the rhombic dodeca hedron predominates. The crystals vary from translucent to transparent, with a vitreous lustre, and are colourless or white, though often tinged with grey, yellow or green. The hardness is as high as 7 on Mohs's scale; specific gravity 3.o.
The characters so far enumerated are strictly in accordance with cubic symmetry, but when a crystal is examined in polar ized light, it will be seen to be doubly refracting, as was first observed by Sir David Brewster in 1821. Thin sections show twin-lamellae, and a division into definite areas which are opti cally biaxial. By cutting sections in suitable directions, it may be proved that a rhombic-dodecahedral crystal is really built up of twelve orthorhombic pyramids, the apices of which meet in the centre and the bases coincide with the dodecahedral faces of the compound (pseudo-cubic) crystal. When the crystals are heated these optical characters change, and at a temperature of 265°C. the crystals suddenly become optically isotropic ; on cooling, however, the complexity of internal structure reappears.
Chemically, Boracite is a magnesium borate and chloride with the formula Mg7C1213,60.. A small amount of iron is sometimes present, and an iron-boracite with half the magnesium replaced by ferrous iron has been called huyssenite. The mineral is in soluble in water, but soluble in hydrochloric acid.
In addition to embedded crystals, a massive variety, known as stassfurtite, occurs as nodules in the salt deposits at Stassfurt in Prussia : that from the carnallite layer is compact, resembling fine-grained marble, and white or greenish in colour, whilst that from the kainite layer is soft, earthy, and yellowish. (L. J. S.)