Anatase is a
metastable mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
form of
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolu ...
(TiO
2) with a
tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other
polymorphs (or mineral forms) of titanium dioxide are known to occur naturally:
brookite,
akaogiite, and
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visib ...
, with rutile being the most common and most
stable
A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals and livestock. There are many different types of stables in use today; the ...
of the bunch. Anatase is formed at relatively low temperatures and found in minor concentrations in
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
and
metamorphic rocks. Thin films of TiO
2-coated glass show
antifogging and
self-cleaning
Government procurement or public procurement is undertaken by the public authorities of the European Union (EU) and its member states in order to award contracts for public works and for the purchase of goods and services in accordance with pri ...
properties under
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
radiation.
Anatase is always found as small, isolated, and sharply developed
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
s, and like rutile, it crystallizes in a
tetragonal system. Anatase is metastable at all temperatures and pressures, with rutile being the equilibrium polymorph. Nevertheless, anatase is often the first titanium dioxide phase to form in many processes due to its lower
surface energy
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less ener ...
, with a transformation to rutile taking place at elevated temperatures. Although the degree of symmetry is the same for both anatase and rutile phases, there is no relation between the interfacial angles of the two minerals, except in the prism-zone of 45° and 90°. The common
octahedral crystal habit
In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities due to l ...
of anatase, with four perfect
cleavage planes, has an angle over its polar edge of 82°9', whereas rutile octahedra only have a polar edge angle of 56°52½'. The steeper angle gives anatase crystals a longer vertical axis and skinnier appearance than rutile, which led French mineralogist
René Just Haüy to name the mineral ''anatase'' in 1801, from the
Greek ''anatasis'' ("extension"). Additional important differences exist between the physical characters of anatase and rutile. For example, anatase is less hard (5.5–6 vs. 6–6.5 on the
Mohs scale
The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by ...
) and less dense (
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its dens ...
about 3.9 vs. 4.2) than rutile. Anatase is also
optically negative, whereas rutile is optically positive. Anatase has a more strongly
adamantine or
metallic
Metallic may be a reference to:
*Metal
*Metalloid, metal-like substance
*Metallic bonding, type of chemical bonding
*Metallicity, in astronomy the proportion of elements other than helium and hydrogen in an object
*Metallic color, a color that g ...
-adamantine
luster than that of rutile as well.
Crystal habit
Two growth
habits of anatase crystals may be distinguished. The more common occurs as simple acute
octahedra 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 () is a commune in the Isère department in southeastern 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 Brianç ...
in
Dauphiné, France, where they are associated with rock-crystal,
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...
, and
axinite in crevices in
granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...
and
mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
schist
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...
. Similar crystals of microscopic size are widely distributed in
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s such as
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
s,
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...
s, and
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. It is the finest grained foliated metamorphic rock. ...
s, from which they may be separated by washing away the lighter constituents of the powdered rock.
The (101) plane of anatase is the most thermodynamically stable surface and thus the most widely exposed
facet in natural and synthetic anatase.
Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit. Their color is honey-yellow to brown. Such crystals closely resemble the mineral
xenotime
Xenotime is a rare-earth phosphate mineral, the major component of which is yttrium orthophosphate ( Y P O4). It forms a solid solution series with chernovite-(Y) ( Y As O4) and therefore may contain trace impurities of arsenic, as well as sili ...
in appearance and were historically thought to be a special form of xenotime, termed ''wiserine''. They occur attached to the walls of crevices in
gneiss
Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...
es in the
Alps
The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
, a well-known locality being the
Binnenthal near
Brig
A brig is a type of sailing vessel defined by its rig: two masts which are both square-rigged. Brigs originated in the second half of the 18th century and were a common type of smaller merchant vessel or warship from then until the latter part ...
in canton
Valais
Valais ( , , ; frp, Valês; german: Wallis ), more formally the Canton of Valais,; german: Kanton Wallis; in other official Swiss languages outside Valais: it, (Canton) Vallese ; rm, (Chantun) Vallais. is one of the 26 cantons forming the S ...
,
Switzerland
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
.
While anatase is not an equilibrium phase of TiO
2, it is metastable near room temperature. At temperatures between 550 and about 1000 °C, anatase converts to rutile. The temperature of this transformation strongly depends on impurities, or
dopants
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. Whe ...
, as well as the morphology of the sample.
Synthetic anatase
Due to its potential application as a
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
, anatase is often prepared synthetically. Crystalline anatase can be prepared in laboratories by chemical methods such as the
sol-gel process. This might be done through controlled
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
of
titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds ...
(TiCl
4) or
titanium ethoxide
Titanium ethoxide is a chemical compound with the formula Ti4(OCH2CH3)16. It is a colorless liquid that is soluble in organic solvents but hydrolyzes readily. It is sold commercially as a colorless solution. Alkoxides of titanium(IV) and zirconium ...
. Often
dopants
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. Whe ...
are included in such synthesis processes to control the morphology, electronic structure, and surface chemistry of an anatase sample.
Alternate and obsolete names
Another name commonly in use for anatase is ''octahedrite'', which 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 of
Le Bourg-d'Oisans
Le Bourg-d'Oisans () is a commune in the Isère department in southeastern 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 Brianç ...
in
Dauphiné
The Dauphiné (, ) is a former province in Southeastern France, whose area roughly corresponded to that of the present departments of Isère, Drôme and Hautes-Alpes. The Dauphiné was originally the Dauphiné of Viennois.
In the 12th centu ...
.
See also
*
Delustrant
*
Titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolu ...
*
Rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visib ...
*
Brookite
*
Akaogiite
*
List of minerals
This is a list of minerals for which there are articles on Wikipedia.
Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various ''species''. Within a m ...
References
{{Titanium minerals
Titanium minerals
Oxide minerals
Tetragonal minerals
Minerals in space group 141