Vauxite is a

phosphate mineral Phosphate minerals contain the tetrahedrally coordinated phosphate (PO43−) anion along sometimes with arsenate (AsO43−) and vanadate (VO43−) substitutions, and chloride (Cl−), fluoride (F−), and hydroxide (OH−) anions that also fit ...

with the chemical formula Fe²⁺ Al₂( P O₄)₂(O H)₂·6(H₂O). It belongs to the laueite – paravauxite group, paravauxite subgroup,American Mineralogist (1968) 53:1025 although Mindat puts it as a member of the vantasselite Al₄(PO₄)₃(OH)₃·9H₂O group. There is no similarity in structure between vauxite and paravauxite Fe²⁺Al₂(PO₄)₂(OH)₂·8H₂O or metavauxite Fe³⁺Al₂(PO₄)₂(OH)₂·8H₂O, even though they are closely similar chemically, and all minerals occur together as secondary minerals. Vauxite was named in 1922 for George Vaux Junior (1863–1927), an American attorney and mineral collector.

Unit cell

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 uncha ...

is P, which means that the only symmetry element for the crystal is a

center of symmetry A fixed point of an isometry group is a point that is a fixed point for every isometry in the group. For any isometry group in Euclidean space the set of fixed points is either empty or an affine space. For an object, any unique centre and, more ...

. The crystal is built up of identical units, called

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 ...

s, which are stacked together so that, in the absence of imperfections, they completely fill the space occupied by the crystal. The unit cell is a

rhomboid Traditionally, in two-dimensional geometry, a rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are non-right angled. A parallelogram with sides of equal length (equilateral) is a rhombus but not a rhomboi ...

(each face is a parallelogram, and opposite pairs of faces are equal) with side lengths a, b and c. The angles between the sides are denoted by the Greek letters α, β and γ, where α is the angle between sides b and c, β between c and a, and γ between a and b. For vauxite, the reported values of these parameters differ slightly from reference to reference, as different researchers have studied different samples, but all agree that a = 9.1 Å, b = 11.6 Å, c = 6 Å, α = 98.3°, β = 92° and γ = 108°, to the accuracy stated. Detailed reported values of the lattice parameters are: * a = 9.13 Å, b = 11.59 Å, c = 6.14 Å, α = 98.3°, β = 92°, γ = 108.4° * a = 9.142 Å, b = 11.599 Å, c = 6.158 Å, α = 98.29°, β = 91.93°, γ = 108.27° Within each unit cell there are two units of the formula Fe²⁺Al₂(PO₄)₂(OH)₂·6H₂O.

Structure

The structure of vauxite is characterised by infinite chains parallel to the c crystal axis. One set of chains is built up of

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

ferrous iron In chemistry, iron(II) refers to the element iron in its +2 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe2+. The adjective ferrous or the prefix ferro- is often used to ...

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 ...

Fe²⁺ or an

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

ion Al in the middle, and an

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 ...

ion O at each of the six vertices. The central ions of these octahedra are alternately Fe and Al, and adjacent octahedra share edges. At each linked edge two oxygen ions are shared between two octahedra, and each octahedron must have two shared edges to form a chain. Parallel to these edge-linked octahedral chains are vertex-linked mixed chains of alternating octahedra and

tetrahedra In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

. The tetrahedra have a

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

ion P in the middle, and oxygen ions O at each of the four vertices, and the octahedra have an aluminium ion Al in the middle surrounded by six oxygen ions O, as in the octahedral chains. At each linked vertex one O is shared between a tetrahedron and an octahedron, and each tetrahedron and octahedron must have two linked vertices to form the mixed chain. Each octahedral chain is flanked by two mixed chains, one on either side, linked through the vertices of the chains, making an infinite triple chain. The triple chains are further interlinked by yet more phosphorus tetrahedra (not the ones in the mixed chains), which share vertices with both kinds of octahedra in the octahedral chains, and with the aluminium octahedra in the mixed chains. Water molecules (H₂O) and

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...

ions (OH) are also incorporated into this chain, giving a complex chain with composition eAl₃(PO₄)₄(OH)₄(OH₂)₂sup>5−. These complex chains, which are parallel to the c crystal axis, are linked in the direction of the a axis by further aluminium octahedra (not the ones in the chains) and in the direction of the b axis by further Fe octahedra, and there are more water molecules within channels in the structure, giving the final formula for vauxite as FeAl₂(PO₄)₂(OH)₂.6H₂O.

Crystal habit

Vauxite crystals are very small and tabular, forming sub-parallel to radial aggregates and nodules. The crystals are flattened parallel to the plane containing the a and c crystal axes, and elongated in the c direction, that is along the length of the chains which are the basis of the structure.

Optical properties

triclinic 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal i ...

mineral, such as vauxite, has all three of its crystal axes of different lengths, and all three interaxial angles of different sizes, with none equal to 60°, 90° or 120°. Consequently, the material is anisotropic, and physical properties, including optical properties, vary with direction.Walter Borchardt-Ott (1993) Crystallography, Springer Verlag The refractive index is the ratio of the speed of light in vacuum to the speed of light through the medium. Since this varies with the

color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...

of the light, a standard color must be chosen when refractive indices are specified. The usual standard is the yellow light from a sodium source, that has wavelength 589.3

nanometers 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...

. For an anisotropic substance the refractive index (for light of a given color) varies with direction, and for vauxite the range is from 1.551 for light travelling parallel to the a axis to 1.562 for light travelling parallel to the c axis. An

optic axis An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propagat ...

is a direction in which light travels through a crystal such that the speed is the same for all directions of polarization for light of any given wavelength (i.e. color). Any direction in an isometric crystal has this property. Trigonal, tetragonal and hexagonal crystals have a single optic axis, parallel to the c crystal axis. They are said to be uniaxial. Triclinic, monoclinic and orthorhombic crystals have two optic axes, and are said to be biaxial. The angle between the two axes is denoted by 2V. Vauxite is biaxial.

Optic sign

Unpolarized light travels unchanged through an isometric crystal, whatever the direction of travel. In uniaxial and biaxial crystals, light travelling in any direction other than parallel to an optic axis is broken into two polarized rays, the ordinary ray and the extraordinary ray. The ordinary ray travels with the same speed no matter what the direction; this is a consequence of the plane in which it is polarized. The plane of polarization of the extraordinary ray is perpendicular to that of the ordinary ray, and in general its speed will be different. For rays travelling along an optic axis the speeds of the ordinary and extraordinary rays are equal. For all other directions in uniaxial and biaxial crystals the speeds are different. The crystal is said to be positive if the ordinary ray has a greater speed than the extraordinary ray, and negative if the reverse is true. Vauxite is biaxial (+).

Dispersion of the optic axes

The refractive index varies with the wavelength (color) of light, so the positions of the optic axes in biaxial crystals, and the angle 2V between them, will change when the color of the incident light is changed. This phenomenon is usually expressed in the form r > v, indicating that 2V is greater for red than for violet light, or vice versa. For vauxite r > v, 2V is greater for red light than for violet light.

Pleochroism

Pleochroism is the phenomenon of crystals appearing to change color as they are rotated in

plane polarized light Polarization ( also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of th ...

.Klein and Hurlbut (1993) Manual of Mineralogy, 21st edition, Wiley This is due to differential absorption of light vibrating in different directions. Isometric crystals cannot be pleochroic. Uniaxial crystals (trigonal, tetragonal or hexagonal) may show two, but not three, different colors as they are rotated, then they are said to be dichroic (two colors). Biaxial crystals may show three different colors, and then are said to be pleochroic (many colors). Vauxite is strongly pleochroic, colorless along X and Z and blue along Y.

Environment

Vauxite is a secondary mineral derived from the alteration of apatite in

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

veins. It is found associated with

wavellite Wavellite is an aluminium basic phosphate mineral with formula Al3( P O4)2(O H, F)3·5 H2O. Distinct crystals are rare, and it normally occurs as translucent green radial or spherical clusters. Discovery and occurrence Wavellite was first d ...

Al₃(PO₄)₂(OH)₃·5H₂O, metavauxite and paravauxite at the type locality, Siglo Veinte Mine (Siglo XX Mine), Llallagua, Rafael Bustillo Province, Potosí Department, Bolivia,The Mineralogical Record 37-2:155 which is the only known occurrence of this mineral. The type material is conserved at the US National Museum of Natural History, Washington DC: #97561, #103542.

References

External links

* Llallagua: http://www.highbeam.com/doc/1G1-145163326.html * George Vaux: http://mineralogicalrecord.com/labels.asp?colid=318&page=1 * JMol: http://rruff.geo.arizona.edu/AMS/viewJmol.php?id=00175 {{Commons category Iron(II) minerals Aluminium minerals Phosphate minerals Triclinic minerals Minerals in space group 2