a : b : c = 0·8416 : 1 : 0·9444.
Common forms—the three pinakoids a {100}, b {010}, and c {001}, prisms m {110}, l {210}, pyramids e {122}, z {122}, and numerous others.
Angles—(100) ∧ (110) = 40° 5´; (001) ∧ (100) = 48° 18´; (001) ∧ (011) = 43° 22´.
The habit is varied; it occurs usually in bipyramids with e and m or prismatic with m, a, and terminating pyramids. Cleavage ∥ m indistinct, ∥ c very poor.
Hardness 51⁄2-6; sp. gr. 3·87-4·01. Lustre metallic. Colour brown to reddish- and yellowish-brown and black. The optical behaviour is interesting. The acute bisectrix is perpendicular to a (100), but while for red light the plane of the optic axes is (001), for blue it is (010); for an intermediate light, therefore (λ = 5550 µµ), the mineral appears uniaxial.
The chief localities are Bourg d’Oisans, Miask, the St. Gothard, the Tyrol, Magnet Cove in Arkansas, and Tremadoc in Wales.
Titanium dioxide can be obtained crystalline by the action of steam on titanium tetrafluoride, TiF₄, at high temperatures; it is stated that by varying the temperature of the reaction, any one of the three crystalline modifications can be obtained.
The only other minerals which need be mentioned in this class (see [list]) are:
Zirkelite, a complicated mixture of oxides, in which thoria, zirconia, and titanium dioxide act as acidic oxides, and
Mackintoshite, a mixture of several oxides, of which those of thorium and uranium are the most important.