[92] Rogers, Amer. J. Sci. 1912, [iv.], 33, 54.
The mineral now comes on the market in commercial quantities; pure zirconia almost entirely free from iron can be obtained by leaching with acids. The pure oxide is extraordinarily refractory, and promises to be of great use for crucibles, furnace linings, etc. (vide [p. 324]).
Rutile.
—Titanium dioxide, TiO₂, occurs crystallised in nature in the three minerals Rutile, Brookite, and Anatase (Octahedrite), which therefore form a trimorphous series. They are all stable minerals, though rutile appears the most stable, being occasionally found in pseudomorphs after the other two. The family is remarkable in that it is not unusual to find two of them occurring together—an uncommon phenomenon with polymorphous minerals.
Rutile often contains small quantities of iron and chromium. The ferriferous varieties are distinguished as Nigrine, which is black, with 2-3 per cent. ferric oxide, and Ilmenorutile, with up to 10 per cent. of ferric oxide, and specific gravity up to 5·13.
Crystal system—tetragonal, holosymmetric; c = 0·6442; (001) ∧ (101) = 32° 47´.
Common forms—prisms a {100}, m {110}, and l {310}; pyramids e {101}, s {111}, and many others. The basal pinakoid c {001} is very rare. Habit, prismatic, with vertical striations; or in slender needles. Twinning very common and varied; usually on the cassiterite law—twin plane e (101)—forming the knee-shaped twins, and irregular rosettes by repetition, and many contact twins. Contact twins on the law—twin plane v (301) are less common.
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]