ANATASE, one of the three mineral forms of titanium dioxide. It is always found as small, isolated and sharply de veloped crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but, although the degree of symmetry is the same for both, there is no relation between the interfacial angles of the two minerals, except, of course, in the prism-zone of 45° and 9o°. There are also important differences between the physical char acters of anatase and rutile; the former is not quite so hard (H.= 54-6) or dense (sp. gr. =3.9) ; it is optically negative, rutile being positive.
Two types or habits of anatase crystals may be distinguished. The commoner occurs as simple acute double pyramids with an indigo-blue to black colour and steely lustre. Crystals of this kind are abundant at Le Bourg d'Oisans in Dauphine, where they are associated with rock-crystal, felspar and axinite in crevices in granite and mica-schist. Similar crystals, but of microscopic size, are widely distributed in sedimentary rocks, such as sand stones, clays and slates, from which they may be separated by washing away the lighter constituents of the powdered rock. Crys tals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit ; the colour is honey-yellow to brown. Such crystals occur attached to the walls of crevices in the gneisses of the Alps, the Binnenthal, Switzerland, being a well-known locality. Another common name for this mineral is octahedrite.