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Anatase

Anatase
Anatase Oisans.jpg
General
Category Oxide minerals
Formula
(repeating unit)
TiO2
Strunz classification 4.DD.05
Crystal system Tetragonal
Crystal class Ditetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space group I41/amd
Unit cell a = 3.7845, c = 9.5143 [Å]; Z = 4
Identification
Formula mass 79.88 g/mol
Color Black, reddish to yellowish brown, dark blue, gray
Crystal habit Pyramidal (crystals are shaped like pyramids), tabular (form dimensions are thin in one direction).
Twinning Rare on {112}
Cleavage Perfect on [001] and [011]
Fracture Subconchoidal
Tenacity Brittle
Mohs scale hardness 5.5–6
Luster Adamantine to splendent, metallic
Streak pale yellowish white
Diaphaneity Transparent to nearly opaque
Specific gravity 3.79–3.97
Optical properties Uniaxial (-), anomalously biaxial in deeply colored crystals
Refractive index nω = 2.561, nε = 2.488
Birefringence δ = 0.073
Pleochroism Weak
References

Anatase is one of the three mineral forms of titanium dioxide, the other two being brookite and rutile. It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but, although the degree of symmetry is the same for both, there is no relation between the interfacial angles of the two minerals, except in the prism-zone of 45° and 90°. The common pyramid of anatase, parallel to the faces of which there are perfect cleavages, has an angle over the polar edge of 82°9', the corresponding angle of rutile being 56°52½'. It was on account of this steeper pyramid of anatase that the mineral was named, by René Just Haüy in 1801, from the Greek anatasis, "extension", the vertical axis of the crystals being longer than in rutile. There are also important differences between the physical characters of anatase and rutile: the former is less hard (5.5–6 vs. 6-6.5 Mohs) and dense (specific gravity about 3.9 vs. 4.2). Also, anatase is optically negative whereas rutile is positive, and its luster is even more strongly adamantine or metallic-adamantine than that of rutile.

Two types or habits of anatase crystals may be distinguished. The more common occurs as simple acute double pyramids with an indigo-blue to black color and steely luster. Crystals of this kind are abundant at Le Bourg-d'Oisans in Dauphiné, where they are associated with rock-crystal, feldspar, and axinite in crevices in granite and mica-schist. Similar crystals, but of microscopic size, are widely distributed in sedimentary rocks, such as sandstones, clays, and slates, from which they may be separated by washing away the lighter constituents of the powdered rock.


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