1911 Encyclopædia Britannica/Sphene

by Leonard James Spencer

SPHENE, a mineral consisting of calcium titano-silicate, CaTiSiO₅, crystallizing in the mono clinic system. The crystals vary considerably in habit, but are generally thin and wedge-shaped; hence the name sphene, from the Greek σφήν (a wedge), given by R. J. Haüy in 1801. The earlier name titanite, given by M. H. Klaproth in 1795, is also in common use. Twinning on the ortho-pinacoid is not uncommon. The colour is green, yellow, brown or black, and the lustre resinous to adamantine; crystals are transparent to opaque. The hardness is 5½, and the specific gravity 3.5. The refractive indices and the optic axial angle vary considerably with the colour of the light: the dispersion of the optic axes is inclined, and the interference figure seen in convergent light between crossed nicols is very characteristic of the mineral. Sphene is sometimes cut as a gem-stone, though it is rather too soft to stand much wear; owing to its high dispersive power it gives brilliant flashes of prismatic colours. As small embedded crystals, sphene has a wide distribution as an accessory constituent of many kinds of igneous rocks (granite, syenite, trachyte, phonolite, &c.), and also of gneiss, schist and crystalline limestone. Sharply-developed, transparent, pale green crystals are frequently associated with adularia, asbestos and quartz in the crystal-lined crevices of the schists of the, Swiss and Tyrolese Alps. Large, rough and dark-coloured crystals are found at Arendal and Kragerö in Norway, and in granular limestone at Diana in New York and Eganville in Ontario. A greyish, compact and impure variety of sphene, known as "leucoxene," frequently occurs in basic igneous rocks as an alteration product of ilmenite and rutile.  (L. J. S.)