On account of the difference in colour and composition, a large number of varieties are distinguished. The ordinary yellow and brown varieties are known indifferently as sphene or titanite. _Ligurite_ has an apple-green colour; _Semeline_ is a greenish form named from a fancied resemblance to flax seed. _Lederite_ is a brown variety of tabular habit; _Greenovite_ is rose-coloured, and contains manganese. _Alshedite_ and _Eucolite-Titanite_ are rich in the trivalent metals; _Grothite_ is a brown variety containing a considerable percentage of ferric iron. _Yttrotitanite_, which contains a high proportion of rare earths, is usually treated as a separate species (see above). _Titanomorphite_ and _Leucoxene_ are white amorphous varieties chiefly produced by alteration of rutile and ilmenite. Titanite is a fairly widespread mineral; as an accessory rock constituent it is common in the massive plutonic rocks in tiny crystals, readily distinguished under the microscope by the high refraction and birefringence, whilst in large embedded crystals it occurs in many granular limestones, and in plutonic acid, as well as in some metamorphic rocks. In good crystals it is found in many parts of Switzerland and the Alps, in Dauphiné, the Tyrol, Piedmont, the Urals, South Norway, and other European localities; it is also widely distributed in the United States and Canada. The mineral is important as a valuable source of titanium. * * * * * The class of Titano-silicates is a very large one, and might be extended almost at will by the inclusion of the numerous silicates which contain titanium. Owing to the frequency with which small quantities of silica are replaced by titanium dioxide, almost all the commoner silicate minerals contain the latter oxide, so that titanium is one of the most widely distributed of the elements. Relatively very few, however, of the titanium-bearing minerals contain the element in considerable quantities, and only two or three have any importance as commercial sources of titanium compounds. Only those additional titano-silicates which contain titanium as an important constituent are mentioned below; short accounts will be found in the alphabetical list. _Johnstrupite_, _Mosandrite_, _Rinkite_, _Rosenbuschite_ and _Tscheffkinite_ are complex titano-silicates containing yttrium or cerium metals. _Astrophyllite_, _Leucosphenite_, _Molengraafite_, _Neptunite_ and _Rhönite_ are complex titano-silicates free from rare earth elements. _Benitoite_ is a simple titano-silicate of barium; _Ænigmatite_ and _Narsarsukite_ contain iron and sodium; _Lorenzenite_ has sodium and zirconium. _Schorlomite_ is a titaniferous garnet. A variety of olivine rich in titanium (_Titanium Olivine_) is also known. (_b_) THE TITANATES