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an·a·tase (ăn'ə-tās', -tāz')  |
A rare blue or light yellow to brown crystalline mineral, the rarest of three forms of titanium dioxide, TiO2, used as a pigment, especially in paint.
[French, from Greek anatasis, extension (from its long crystals), from anateinein, anata-, to extend : ana-, ana- + teinein, to stretch.]
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Environment
In seams and silicate veins, probably formed at fairly low temperatures.
Crystal descriptionSeveral habits, but always crystallized. Steeply pyramidal, pseudo-octahedral or tabular, often very complexly modified.
Physical propertiesBlue, light yellow to brown. Luster adamantine to submetallic; hardness 5Ɖ-6; specific gravity 3.8-3.9; streak white; fracture subconchoidal; cleavage perfect basal and pyramidal. Brittle; translucent to transparent.
CompositionTitanium oxide (60.0% Ti, 40.0% O) like rutile, except that its atomic structure, and therefore its crystal form, is different.
TestsSame as for rutile (p. 151).
Distinguishing characteristicsUsually the crystal form is sufficient. Brown equidimensional crystals might be confused with an octahedral mineral such as microlite, which is generally far heavier. It is safest to get the titanium coloration test (see ilmenite, p. 149).
OccurrenceAnatase is most frequent in vein- or fissure-type alpine deposits. Of value only to collectors, because of its rarity, but holds great scientific interest. Steep blue-black bipyramids lie on quartz in the French Alpine region and in northern Norway. Modified pseudo-octahedral brown crystals occur on the calcareous gneiss in fissures in the Binnatal area, Switzerland, and at Spissen, where some of the largest known crystals occur: steep ditetragonal bipyramids as much as Ɖ in. (1 cm) tall. Three-cm crystals have been found at Grieserntal. It is found in Brazil on some of the quartz of the veins of the Diamantina district, often altered to rutile if the crystals were not encased in and protected by the quartz. Waterworn, gemmy, tiny deep blue tabular crystals have come from diamond washing in Minas Gerais, Brazil. Anatase is not common in the U.S. The largest crystals (blue) have been found in Gunnison Co., Colorado. Small steep bipyramids were found in calcite-filled quartz veins in a quarry at Somerville, Massachusetts. Waterworn crystals were found in gold washings in North Carolina, and minute steep bipyramids on mica flakes in Lincoln Co., North Carolina.
RemarksAnatase is our second of three titanium oxides (with rutile and brookite), all chemically alike but distinguished by their crystal symmetries. Probably rarest of the three, and supposedly the one deposited at the lowest temperature. It is also the most readily altered of the titanium oxides.
| Wikipedia: Anatase |
| Anatase | |
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| General | |
|---|---|
| Category | mineral |
| Chemical formula | TiO2 |
| Identification | |
| Molar mass | 79.88 |
| Color | Black, Reddish brown, Yellowish brown, Dark blue, Gray. |
| Crystal habit | Pyramidal - Crystals are shaped like pyramids. Tabular - Form dimensions are thin in one direction. |
| Crystal system | Tetragonal (4/m 2/m 2/m) Space Group: I 41/amd |
| Cleavage | [101] Perfect, [001] Distinct |
| Fracture | Conchoidal - Fractures developed in brittle materials characterized by smoothly curving surfaces, (e.g. quartz). |
| Mohs Scale hardness | 5.5-6 |
| Luster | Adamantine - Resinous |
| Streak | pale yellowish white |
| Specific gravity | 3.9 |
| Optical properties | Uniaxial (-), e=2.488, w=2.561 |
| Other characteristics | non-radioactive, non-magnetic, Non-fluorescent. |
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, of course, 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 RJ 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 not quite so hard (H=5½–6) or dense (specific gravity 3.9); it is optically negative, rutile being positive; and its lustre is even more strongly adamantine or metallic-adamantine than that of rutile.
Two types or habits of anatase crystals may be distinguished. The commoner occurs as simple acute double pyramids with an indigo-blue to black colour and steely lustre. 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.
Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit; the colour is honey-yellow to brown. Such crystals closely resemble xenotime in appearance and, indeed, were for a long time supposed to belong to this species, the special name wiserine being applied to them. They occur attached to the walls of crevices in the gneisses of the Alps, the Binnenthal near Brig in canton Valais, Switzerland, being a well-known locality.
When heated above 915°C, anatase is converted into rutile, changing in specific gravity to 4.1; naturally occurring pseudomorphs of rutile after anatase are also known. Crystals of anatase can be prepared in laboratories by chemical methods such as sol-gel processing. Examples include controlled hydrolysis of titanium tetrachloride (TiCl4) or titanium alkoxides. Such synthetic anatases are of considerable interest for photocatalysis and for Graetzel-cell photovoltaics.
Another name commonly in use for this mineral is octahedrite, a name which, indeed, is earlier than anatase, and given because of the common (acute) octahedral habit of the crystals. Other names, now obsolete, are oisanite and dauphinite, from the well-known French locality.
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Rock & Mineral Guide. Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Anatase". Read more |
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