Brookite
Encyclopedia
Brookite is orthorhombic
, and one of the four naturally occurring polymorphs
(minerals with the same composition but different structure) of titanium dioxide
, TiO2, approved by the International Mineralogical Association
(IMA). The others are akaogiite (monoclinic
), anatase
(tetragonal
) and rutile
(tetragonal
). Brookite is rare compared to anatase and rutile and, like these forms, it exhibits photocatalytic
activity. Brookite has a larger cell
volume than either anatase or rutile, with 8 TiO2 groups per unit cell, compared with 4 for anatase and 2 for rutile. Iron
Fe, tantalum
Ta and niobium
Nb are common impurities.
It was named in 1825 for Henry James Brooke
(1771–1857), an English crystallographer, mineralogist and wool trader.
Arkansite is a variety of brookite from Arkansas
, USA, that is also found in the Murunskii Massif, in the Eastern Siberia
n region of Russia, where many other unusual minerals occur.
At temperatures above about 750 °C, brookite will revert to the rutile structure.
dipyramidal crystal class
2/m 2/m 2/m (also designated mmm). The space group
is Pcab and the unit cell
parameters are a = 5.4558 Å, b = 9.1819 Å and c = 5.1429 Å. The formula
is TiO2, with 8 formula units per unit cell (Z = 8).
with a titanium
ion
at the center and oxygen
ions at each of the six vertices. Each octahedron shares three edges with adjoining octahedra, forming an orthorhombic
structure.
, pseudo-hexagonal or prismatic
. Brookite and rutile
may grow together in an epitaxial relationship.
Brookite is usually brown in color, sometimes yellowish or reddish brown or even black, with a submetallic luster
. It is opaque to translucent, transparent in thin fragments and yellowish brown to dark brown in transmitted light.
, as are all orthorhombic minerals, and it is biaxial (+)
. Refractive indices
are very high, above 2.5, which is even higher than diamond
at 2.42. For comparison, ordinary window glass has a refractive index of about 1.5.
Brookite exhibits very weak pleochroism
, yellowish, reddish and orange to brown. It is neither fluorescent
nor radioactive
.
and poor cleavage
in one direction parallel to the c crystal axis
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 of 5½ to 6, between apatite
and feldspar
. This is the same hardness as anatase and a little less than that of rutile (6 to 6½). The specific gravity
is 4.08 to 4.18, between that of anatase at 3.9 and rutile at 4.2.
in gneiss
and schist
; it is also a common detrital mineral.
Associated minerals include its polymorphs
anatase
and rutile
, and also titanite
, orthoclase
, quartz
, hematite
, calcite
, chlorite
and muscovite
.
The type locality is Twll Maen Grisial, Fron Olau, Prenteg
, Gwynedd, Wales
, UK, and in 2004 fine brookite crystals were found at Kharan
, in Balochistan, Pakistan, together with brookite and rutile inclusions
in quartz
.
Orthorhombic crystal system
In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
, and one of the four naturally occurring polymorphs
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
(minerals with the same composition but different structure) 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...
, TiO2, approved by the International Mineralogical Association
International Mineralogical Association
The International Mineralogical Association is an international group of 38 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 4000 plus known mineral species...
(IMA). The others are akaogiite (monoclinic
Monoclinic crystal system
In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. They form a rectangular prism with a...
), anatase
Anatase
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,...
(tetragonal
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...
) 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:...
(tetragonal
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...
). Brookite is rare compared to anatase and rutile and, like these forms, it exhibits photocatalytic
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,...
activity. Brookite has a larger cell
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
volume than either anatase or rutile, with 8 TiO2 groups per unit cell, compared with 4 for anatase and 2 for rutile. Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
Fe, tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
Ta and niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
Nb are common impurities.
It was named in 1825 for Henry James Brooke
Henry James Brooke
Henry James Brooke , was an English crystallographer.Brooke was the son of a broadcloth manufacturer, born in Exeter on 25 May 1771, studied for the bar, but went into business in the Spanish wool trade, South American mining companies, and the London Life Assurance Association successively. He...
(1771–1857), an English crystallographer, mineralogist and wool trader.
Arkansite is a variety of brookite from Arkansas
Arkansas
Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...
, USA, that is also found in the Murunskii Massif, in the Eastern Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
n region of Russia, where many other unusual minerals occur.
At temperatures above about 750 °C, brookite will revert to the rutile structure.
Unit cell
Brookite belongs to the orthorhombicOrthorhombic crystal system
In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
dipyramidal crystal class
Crystallographic point group
In crystallography, a crystallographic point group is a set of symmetry operations, like rotations or reflections, that leave a central point fixed while moving other directions and faces of the crystal to the positions of features of the same kind...
2/m 2/m 2/m (also designated mmm). The space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
is Pcab and the unit cell
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
parameters are a = 5.4558 Å, b = 9.1819 Å and c = 5.1429 Å. The formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
is TiO2, with 8 formula units per unit cell (Z = 8).
Structure
The brookite structure is built up of distorted octahedraOctahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
with a titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
at the center and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
ions at each of the six vertices. Each octahedron shares three edges with adjoining octahedra, forming an orthorhombic
Orthorhombic crystal system
In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
structure.
Appearance
Crystals are typically tabular, elongated and striated parallel to their length. They may also be pyramidalPyramid (geometry)
In geometry, a pyramid is a polyhedron formed by connecting a polygonal base and a point, called the apex. Each base edge and apex form a triangle. It is a conic solid with polygonal base....
, pseudo-hexagonal or prismatic
Prism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...
. Brookite 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:...
may grow together in an epitaxial relationship.
Brookite is usually brown in color, sometimes yellowish or reddish brown or even black, with a submetallic luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
. It is opaque to translucent, transparent in thin fragments and yellowish brown to dark brown in transmitted light.
Optical properties
Brookite is doubly refractingBirefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
, as are all orthorhombic minerals, and it is biaxial (+)
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
. Refractive indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
are very high, above 2.5, which is even higher than diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
at 2.42. For comparison, ordinary window glass has a refractive index of about 1.5.
Brookite exhibits very weak pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
, yellowish, reddish and orange to brown. It is neither fluorescent
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
nor radioactive
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
.
Physical properties
Brookite is a brittle mineral, with a subconchoidal to irregular fractureFracture (mineralogy)
In the field of mineralogy, fracture is a term used to describe the shape and texture of the surface formed when a mineral is fractured. Minerals often have a highly distinctive fracture, making it a principal feature used in their identification....
and poor cleavage
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
in one direction parallel to the c crystal axis
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
and traces of cleavage in a direction perpendicular to both the a and the b crystal axes. Twinning
Crystal twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....
is uncertain. The mineral has a Mohs hardness of 5½ to 6, between apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
and feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. This is the same hardness as anatase and a little less than that of rutile (6 to 6½). The 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...
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 veinsVein (geology)
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation...
in 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:...
and 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...
; it is also a common detrital mineral.
Associated minerals include its polymorphs
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
anatase
Anatase
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,...
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:...
, and also titanite
Titanite
Titanite, or sphene , is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present...
, orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
and muscovite
Muscovite
Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
.
The type locality is Twll Maen Grisial, Fron Olau, Prenteg
Prenteg
Prenteg is a hamlet that lies from Porthmadog, Wales, between Tremadog and Beddgelert.-See also:*The July 2010 wind storms in Global storm activity of 2010....
, Gwynedd, Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
, UK, and in 2004 fine brookite crystals were found at Kharan
Kharan District
Kharan is a district in the north-west of Balochistan province of Pakistan. Kharan was notified as a district in 1951 and the Deputy Commissioner’s office started functioning on March 15, 1952. The Deputy Commissioner directly supervises all activities in the district and the functions of all...
, in Balochistan, Pakistan, together with brookite and rutile inclusions
Inclusion (mineral)
In mineralogy, an inclusion is any material that is trapped inside a mineral during its formation.In gemology, an inclusion is a characteristic enclosed within a gemstone, or reaching its surface from the interior....
in quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
.
See also
- A list of minerals with associated Wikipedia articles
- A comprehensive list of minerals
- List of minerals named after people