Cleavage ∥ _a_ (100) and _m_ (110), distinct. Hardness 6-6¹⁄₂; sp. gr. 4·18-4·25, and up to 5·2 if much iron is present. Colour reddish-brown to black, with good metallic lustre; transparent to opaque. The refraction and double refraction are very high--ω = 2·6158, ε = 2·9029 for sodium light--and allow the crystals to be readily distinguished in rock-sections. The mineral is insoluble in acids, but can be dissolved after fusion with alkalies or alkali carbonates. Rutile is a member of the isomorphous series, cassiterite, zircon, etc. (see under Thorite), and in particular it has the colour, appearance, and twinning of cassiterite, from which, however, it is readily distinguished by its lower specific gravity. In this connection it is interesting to note that an apparently pure specimen, quite free from inclusions, was found (1904) to contain 1·7 per cent. of tin dioxide.[93] [93] Friedel et Grandjean, _Bull. Soc. franc. Min._ 1909, ~32~, 52. As an accessory rock mineral, and also as an important constituent of many sands, rutile is of very wide distribution. It occurs, usually imbedded in quartz or felspar, in many granites, syenites, gneisses, slates, and allied rocks; in acicular crystals penetrating quartz it forms the ‘Veneris Crinis’ of Pliny. At Risör and other localities in Norway, it is found in the massive form, and it is largely worked at Risör as a source of titanium. It occurs in all the countries of Europe, and largely in America. Arendal, Kragerö, and Risör, in Norway, the Binnenthal, the Urals, the St. Gothard, Castile, Magnet Cove in Arkansas, Alexander Co. in N. Carolina, Barre and Shelburne in Massachusetts, and Chester Co. in Pennsylvania are the chief localities. It was in this mineral that the element titanium was first recognised by Klaproth (1795). ~Anatase~ (Octahedrite) is the second crystalline modification of titanium dioxide. Tetragonal _c_ = 1·7771. (001) ∧ (101) = 60° 38´, (111) ∧ (11̅1) = 82° 9´. Common forms--Prisms _a_ {100} and _m_ {110}, pyramids _p_ {111}, _e_ {101}, and many other complex forms; the basal plane _c_ {001} is occasionally found. Habit usually octahedral, with _p_ or _v_ prominent; sometimes tabular with _c_, more rarely prismatic with _a_ well developed. Cleavage ∥ _c_ and _p_ perfect. Hardness 5¹⁄₂-6; sp. gr. 3·82-3·95, usually increasing after heating. Lustre adamantine, so splendent that in Brazil detached crystals have been mistaken for diamonds. Colour, some shade of bluish-black to brown; by transmitted light, greenish-yellow. Transparent to opaque. Double refraction negative, strong; for sodium light ω = 2·554, ε = 2·493.