Anatase
Encyclopedia
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 crystal
s, 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.
of anatase crystals may be distinguished. The commoner 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 rock
s, such as sandstone
s, clay
s, and slate
s, 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 color 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 gneiss
es of the Alps
, the Binnenthal near Brig
in canton Valais
, Switzerland
, being a well-known locality. Naturally occurring pseudomorphs of rutile after anatase are also known.
While anatase is not an equilibrium phase of TiO2, it is kinetically stabilized. At temperatures between 550 and about 1000 °C, anatase transforms to the equilibrium rutile phase, increasing its specific gravity to 4.2. The temperature of this transformation strongly depends on the impurities or dopants present in the material as well as on the morphology of the sample.
(TiCl4) or titanium alkoxide
s. Such synthetic anatases are of considerable interest for photocatalysis
and for Graetzel-cell photovoltaics.
, 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.
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
forms of titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
, the other two being brookite
Brookite
Brookite is orthorhombic, and one of the four naturally occurring polymorphs of titanium dioxide, TiO2, approved by the International Mineralogical Association . The others are akaogiite , anatase and rutile...
and rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
. It is always found as small, isolated and sharply developed crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system
Tetragonal crystal system
In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
; 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
René Just Haüy
René Just Haüy – 3 June 1822 in Paris) was a French mineralogist, commonly styled the Abbé Haüy after he was made an honorary canon of Notre Dame. He is often referred to as the "Father of Modern Crystallography." -Biography:...
in 1801, from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
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
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
) and dense (specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
about 3.9 vs. 4.2). Also, anatase is optically negative whereas rutile is positive, and its luster is even more strongly adamantine
Adamantine
Adamantine is a mineral, often referred to as adamantine spar. It is a silky brown form of corundum. It has a Mohs rating of 9.Adamantine is also used as an adjective to refer to non-metallic, brilliant light reflecting and transmitting properties, known as adamantine luster...
or metallic-adamantine than that of rutile.
Crystal habit
Two types or habitsCrystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
of anatase crystals may be distinguished. The commoner 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
Le Bourg-d'Oisans
Le Bourg-d'Oisans is a commune in the Isère department in south-eastern France.It is located in the Oisans region of the French Alps. Le Bourg-d'Oisans is located in the valley of the Romanche river, on the road from Grenoble to Briançon, and on the south side of the Col de la Croix de Fer...
in Dauphiné
Dauphiné
The Dauphiné or Dauphiné Viennois is a former province in southeastern France, whose area roughly corresponded to that of the present departments of :Isère, :Drôme, and :Hautes-Alpes....
, where they are associated with rock-crystal, feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, and axinite
Axinite
Axinite is a brown to violet-brown, or reddish-brown bladed group of minerals composed of calcium aluminium boro-silicate, 3Al2BO3Si4O12OH...
in crevices in granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
and mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
-schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
. Similar crystals, but of microscopic size, are widely distributed in sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s, such as sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s, clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
s, and slate
Slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
s, 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 color is honey-yellow to brown. Such crystals closely resemble xenotime
Xenotime
Xenotime is a rare earth phosphate mineral, whose major component is yttrium orthophosphate . It forms a solid solution series with chernovite- and therefore may contain trace impurities of arsenic, as well as silicon dioxide and calcium...
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 gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es of the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
, the Binnenthal near Brig
Brig, Switzerland
Brig, officially Brig-Glis is a municipality in the district of Brig in the canton of Valais in Switzerland.The current municipality was formed in 1972 through the merger of Brig , Brigerbad and Glis.-History:...
in canton Valais
Valais
The Valais is one of the 26 cantons of Switzerland in the southwestern part of the country, around the valley of the Rhône from its headwaters to Lake Geneva, separating the Pennine Alps from the Bernese Alps. The canton is one of the drier parts of Switzerland in its central Rhône valley...
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
, being a well-known locality. Naturally occurring pseudomorphs of rutile after anatase are also known.
While anatase is not an equilibrium phase of TiO2, it is kinetically stabilized. At temperatures between 550 and about 1000 °C, anatase transforms to the equilibrium rutile phase, increasing its specific gravity to 4.2. The temperature of this transformation strongly depends on the impurities or dopants present in the material as well as on the morphology of the sample.
Synthetic anatase
Crystals of anatase can be prepared in laboratories by chemical methods such as sol-gel processing. Examples include controlled hydrolysis of titanium tetrachlorideTitanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile...
(TiCl4) or titanium alkoxide
Alkoxide
An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands...
s. Such synthetic anatases are of considerable interest for photocatalysis
Photocatalysis
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the ability of the catalyst to create electron–hole pairs,...
and for Graetzel-cell photovoltaics.
Alternate and obsolete names
Another name commonly in use for this mineral is octahedriteOctahedrite
Octahedrites are a class of iron meteorites within the structural classification. They are the most common class of iron meteorites.They are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content....
, 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.