Brookite is the orthorhombic variant 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 insolubl ...

(TiO₂), which occurs in four known natural polymorphic forms (minerals with the same composition but different structure). The other three of these forms are

akaogiite Akaogiite ( IMA symbol Aka) is an exceedingly rare mineral, one of the natural forms of titanium dioxide (TiO2). It is a high-pressure polymorph of TiO2, along with anatase, brookite and another high-pressure phase called "TiO2 II". Rutile is the ...

(

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

anatase Anatase is a metastable mineral form of titanium dioxide (TiO2) 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) ...

(

tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. 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 ...

) and

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer Polymorphism (materials science), polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest ...

(

). Brookite is rare compared to anatase and rutile and, like these forms, it exhibits

photocatalytic In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the abi ...

activity. Brookite also has a larger

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

volume than either anatase or rutile, with 8 TiO₂ groups per unit cell, compared with 4 for anatase and 2 for rutile.Anatase and Brookite
. Wikis.lib.ncsu.edu (2007-05-08). Retrieved on 2011-10-14.

Iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

(Fe),

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that is ...

(Ta) and

niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has sim ...

(Nb) are common impurities in brookite. Brookite was named in 1825 by French mineralogist Armand Lévy for

Henry James Brooke Henry James Brooke (1771–1857) was an English crystallography, crystallographer. Life Brooke was the son of a broadcloth manufacturer, born in Exeter on 25 May 1771. He studied for the bar, but went into business in the Spanish wool trade, Sout ...

(1771–1857), an English crystallographer, mineralogist and wool trader. Arkansite is a variety of brookite from

Magnet Cove, Arkansas Magnet Cove is a census-designated place (CDP) and former town in Hot Spring County, Arkansas, United States. It is located in the Ouachita Mountains southeast of Hot Springs, on Arkansas Highway 51 north of U.S. Highway 270. As of the 2020 censu ...

, US. It is also found in the

Murun Massif Murun (russian: Мурун) is a mountain in the Olyokma-Chara Plateau, at the border of Irkutsk Oblast and Yakutia, Russian Federation. Geography A high summit is the highest point of the Murun Massif in the Olyokma-Chara Plateau, part of th ...

on the

Olyokma-Chara Plateau The Olyokma-Chara Plateau ( rus, Олёкмо-Чарское плоскогорье; sah, Өлүөхүмэ-Чаара хаптал хайалаах сир) is a mountainous area in the Sakha Republic and Irkutsk Oblast, Far Eastern Federal Di ...

of Eastern

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...

, Russia, part of the

Aldan Shield Aldan Shield (russian: Алданский щит) is a shield in Siberia. It is an exposed basement of the Siberian Craton. Together with the Anabar Shield further to the northwest, the Aldan Shield is one of the main features of the craton. T ...

.Arkansite on Mindat
/ref> At temperatures above about 750 °C, brookite will revert to the rutile structure.
Galleries.com. Retrieved on 2011-10-14.

Unit cell

Brookite belongs to the orthorhombic dipyramidal

crystal class In crystallography, a crystallographic point group is a set of symmetry operations, corresponding to one of the point groups in three dimensions, such that each operation (perhaps followed by a translation) would leave the structure of a crystal un ...

2/m 2/m 2/m (also designated mmm). The

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...

is Pcab and the

unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...

parameters are a = 5.4558 Å, b = 9.1819 Å and c = 5.1429 Å. The

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...

is TiO₂, with 8 formula units per unit cell.

Structure

The brookite structure is built up of distorted

octahedra In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

with a

titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

at the center and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

ions at each of the six vertices. Each octahedron shares three edges with adjoining octahedra, forming an orthorhombic structure.The Crystal Structure of Brookite
paulingblog.wordpress.com. 12 January 2010

Appearance

Brookite crystals are typically tabular, elongated and striated parallel to their length. They may also be

pyramidal A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilater ...

, pseudo-

hexagon In geometry, a hexagon (from Ancient Greek, Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple polygon, simple (non-self-intersecting) hexagon is 720°. Regular hexa ...

al or

prismatic An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...

. Brookite and

may grow together in an

epitaxial Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...

relationship. Brookite is usually brown in color, sometimes yellowish or reddish brown, or even black. Beautiful, deep red crystals (seen above-right) similar to

pyrope The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for ''fire'' and ''e ...

and almandite garnet are also known. Brookite displays a submetallic luster. It is opaque to translucent, transparent in thin fragments and yellowish brown to dark brown in transmitted light.

Optical properties

Brookite is doubly refracting, as are all orthorhombic minerals, and it is biaxial (+).

Refractive indices In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

are very high, above 2.5, which is even higher than

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...

at 2.42. For comparison, ordinary window glass has a refractive index of about 1.5. Brookite exhibits very weak

pleochroism Pleochroism (from Greek πλέων, ''pléōn'', "more" and χρῶμα, ''khrôma'', "color") is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light. Backgroun ...

, yellowish, reddish and orange to brown. It is neither

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

nor

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

Physical properties

Brookite is a

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...

mineral, with a subconchoidal to irregular

fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...

and poor cleavage in one direction parallel to the ''c''

crystal axis In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns t ...

and traces of cleavage in a direction perpendicular to both the ''a'' and the ''b'' crystal axes. Twinning is uncertain. The mineral has a

Mohs hardness 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 th ...

of to 6, between

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

and

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) feldsp ...

. This is the same hardness as anatase and a little less than that of rutile (6 to ). The

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 (molecule), wa ...

is 4.08 to 4.18, between that of anatase at 3.9 and rutile at 4.2.

Occurrence and associations

Brookite is an accessory mineral in alpine veins 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 an ...

and

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 o ...

; it is also a common detrital mineral. Associated minerals include its polymorphs

and

, and also

titanite Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals ...

orthoclase Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles t ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...

and

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...

. The type locality is Twll Maen Grisial, Fron Olau,

Prenteg Prenteg is a hamlet that lies from Porthmadog, Wales, between Tremadog and Beddgelert Beddgelert () is a village and community in the Snowdonia area of Gwynedd, Wales. The population of the community taken at the 2011 census was 455, a ...

Gwynedd Gwynedd (; ) is a county and preserved county (latter with differing boundaries; includes the Isle of Anglesey) in the north-west of Wales. It shares borders with Powys, Conwy County Borough, Denbighshire, Anglesey over the Menai Strait, and C ...

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

. In 2004, brookite crystals were found in the Kharan, in

Balochistan Balochistan ( ; bal, بلۏچستان; also romanised as Baluchistan and Baluchestan) is a historical region in Western and South Asia, located in the Iranian plateau's far southeast and bordering the Indian Plate and the Arabian Sea coastline. ...

, Pakistan.

References

External links

Brookite structure

JMol
{{Titanium minerals Titanium minerals Oxide minerals Orthorhombic minerals Minerals in space group 61 Polymorphism (materials science)