Talmessite is a

hydrated In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

arsenate The arsenate is an ion with the chemical formula . Bonding in arsenate consists of a central arsenic atom, with oxidation state +5, double bonded to one oxygen atom and single bonded to a further three oxygen atoms. The four oxygen atoms orien ...

, often with significant amounts of

cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

. It was named in 1960 for the type locality, the Talmessi mine,

Anarak Anarak (; ) is a city in, and the capital of, Anarak District of Nain County, Isfahan province, Iran. Demographics Language The people of Anarak speak a dialect called Anaraki. This language is spoken in the city and localities within of ...

district, Iran. It forms a series with β-Roselite, where cobalt replaces some of the magnesium, and with gaitite, where

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

replaces the magnesium. All these minerals are members of the fairfieldite group. Talmessite is dimorphic with wendwilsonite (which is not a member of this group).

Members of the fairfieldite group

* Cassidyite *

Collinsite Collinsite is a mineral with chemical formula . It was discovered in British Columbia, Canada, and formally described in 1927. It was named in honor of William Henry Collins (1878–1937), director of the Geological Survey of Canada. There are thr ...

* Fairfieldite * Gaitite * Messelite * Nickeltalmessite * Parabrandtite * unnamed -analogue of parabrandtite) * β-Roselite * Talmessite

Crystallography

The formula for talmessite is Ca₂Mg(AsO₄)₂.2H₂O. It is a

triclinic class=skin-invert-image, 180px, Triclinic (a ≠ b ≠ c ≠ a and α, β, γ, 90° pairwise different) In crystallography, the triclinic (or anorthic) crystal system is one of the seven crystal systems. A crystal system is described by three b ...

mineral,

crystal class In crystallography, a crystallographic point group is a three-dimensional point group whose symmetry operations are compatible with a three-dimensional crystallographic lattice. According to the crystallographic restriction it may only contain on ...

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

P. There is one formula unit per

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 In mathematics, a unit vector i ...

(Z = 1) and the unit cell parameters are variously given as a = 5.87 Å, b = 6.94 Å, c = 5.53 Å, α = 97.3°, β = 108.7°, γ = 108.1°, or a = 5.89 Å, b = 7.69 Å, c = 5.56 Å, α = 112.633°, β = 70.817°, γ = 119.417°,. These values give a calculated

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

varying from 3.42 to 3.63.
The structure is dominated by chains of

tetrahedral In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...

AsO₄ and

octahedral In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...

cation-O₄(H₂O)₂)groups that parallel the c

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

. The octahedral are compressed, resulting in chain disorder

Appearance

Talmessite occurs as

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

crystals to 3 mm, as radiating fibrous aggregates or as fine crystalline aggregates; it may also be stalactitic or in crusts. Pure talmessite is white or colourless, and colourless in

transmitted light Electromagnetic radiation can be affected in several ways by the medium in which it propagates. It can be scattered, absorbed, and reflected and refracted at discontinuities in the medium. This page is an overview of the last 3. The t ...

, but

-rich varieties are pale green and

-rich varieties may be brownish, pink or the purple colour typical of many cobalt minerals. The streak is white and crystals are transparent to translucent with a

vitreous lustre Lustre (Commonwealth English) or luster (American English; see spelling differences) is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin ''lux'', meaning "light", and general ...

Physical properties

Talmessite is a moderately hard mineral, with

Mohs hardness The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fair ...

5, harder than

fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs scal ...

but not as hard as

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

. The

calculated from 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 betwe ...

and the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

dimensions is 3.49, but the measured value is less for ordinary talmessite, at 3.42 and more for the cobaltoan variety, at 3.57. The mineral displays polysynthetic twinning. It is not

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 conside ...

and it loses

water of crystallisation In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions. In some contexts, water of crystallization is ...

at 450^o.

Optical properties

Triclinic class=skin-invert-image, 180px, Triclinic (a ≠ b ≠ c ≠ a and α, β, γ, 90° pairwise different) In crystallography, the triclinic (or anorthic) crystal system is one of the seven crystal systems. A crystal system is described by three b ...

crystals (and

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...

and

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

crystals) have two directions in which light travels with zero

birefringence Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...

; these directions are called the optic axes, and the crystal is said to be biaxial. Talmessite is triclinic, so it is biaxial.
The speed of a ray of light travelling through the crystal differs with direction. The direction of the fastest ray is called the X direction and the direction of the slowest ray is called the Z direction. X and Z are perpendicular to each other, and a third direction Y is defined as perpendicular to both X and Z; light travelling along Y has an intermediate speed. Refractive index is

inversely proportional In mathematics, two sequences of numbers, often experimental data, are proportional or directly proportional if their corresponding elements have a constant ratio. The ratio is called ''coefficient of proportionality'' (or ''proportionality ...

to speed, so the refractive indices for the X, Y and Z directions increase from X to Z.Klein and Hurlbut (1993) Manual of Mineralogy 21st Edition. Wiley The refractive indices are n_α = 1.672, n_β = 1.685 and n_γ = 1.698 with slightly higher values for the cobaltoan variety of n_α = 1.695 and n_γ = 1.73 (n_β not specified). The maximum birefringence δ is the difference between the highest and lowest refractive index; for talmessite δ = 0.026.
The angle between the two optic axes is called the optic angle, 2V, and it is always acute, and bisected either by X or by Z. If Z is the bisector then the crystal is said to be positive, and if X is the bisector it is said to be negative. Talmessite may be biaxial (–) or biaxial (+). The measured value of 2V is about 90^o. Also 2V can be calculated from the values of the refractive indices, giving a value of 88°
2V depends on the refractive indices, but refractive index varies with

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

, and hence with

colour Color (or colour in Commonwealth English; see spelling differences) is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorp ...

. So 2V also depends on the colour, and is different for red and for violet light. This effect is called dispersion of the optic axes, or just dispersion (not to be confused with

chromatic dispersion Dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Sometimes the term chromatic dispersion is used to refer to optics specifically, as opposed to wave propagation in general. A medium having this commo ...

). If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For talmessite dispersion is strong, with r > v. The cobaltoan variety is

pleochroic Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light. Etymology The roots of the word are from Greek (). It was first made compound in the German term ...

colourless to pale rose.

Occurrence

Talmessite is a rare

secondary mineral A primary mineral is any mineral formed during the original crystallization of the host Igneous rock, igneous Primary Rock, primary rock and includes the essential mineral(s) used to classify the rock along with any accessory minerals. In ore deposi ...

formed typically in the oxidized zone of some

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 ...

mineral deposits, as an alteration product of

realgar Realgar ( ), also known as arsenic blende, ruby sulphur or ruby of arsenic, is an arsenic sulfide mineral with the chemical formula α-. It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, ...

orpiment Orpiment, also known as ″yellow arsenic blende″ is a deep-colored, orange-yellow arsenic sulfide mineral with formula . It is found in volcanic fumaroles, low-temperature hydrothermal veins, and hot springs and may be formed through sublimatio ...

, or Cu–Ni

arsenide In chemistry, an arsenide is a compound of arsenic with a less electronegative element or elements. Many metals form binary compounds containing arsenic, and these are called arsenides. They exist with many Stoichiometry, stoichiometries, and in t ...

Cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...

-rich varieties are found in the oxidised zone of cobalt arsenide deposits. It occurs associated with gaitite,

erythrite Erythrite, also known as red cobalt, previously '' cobalt ochre'Krivovichev V. G.'' Mineralogical glossary. Scientific editor A. G. Bulakh. — St.Petersburg: St.Petersburg Univ. Publ. House. 2009. — 556 p. — ISBN 978-5-288-04863-0 is a ...

annabergite Annabergite is an arsenate mineral consisting of a hydrous nickel arsenate. It is considered a member of the vivianite group and known for its ability to form crystals in a characteristic apple-green color. History Annabergite has been kn ...

picropharmacolite Picropharmacolite, Ca4Mg(AsO3OH)2(AsO4)2·11H2O, is a rare arsenate mineral. It was named in 1819 from the Greek for bitter, in allusion to its magnesium content, and its chemical similarity to pharmacolite. The mineral irhtemite, Ca4Mg(AsO3OH ...

pharmacolite Pharmacolite is an uncommon calcium arsenate mineral with formula CaHAsO4·2(H2O). It occurs as soft, white clusters of fibrous crystals and encrustations which crystallize in the monoclinic system. It is the arsenate analogue of the sulfate minera ...

austinite Austinite is a member of the adelite-descloizite group, adelite subgroup, the zinc (Zn) end member of the copper-Zn series with conichalcite. It is the zinc analogue of cobaltaustinite and nickelaustinite. At one time “brickerite” was thoug ...

baryte Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate (Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

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 ...

and dolomite. At the type locality it is associated with

and dolomite.

Localities

The type locality is the Talmessi Mine,

District, Nain County, Esfahan Province, Iran, and

type material In biology, a type is a particular wikt:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to ancho ...

is conserved at the

École nationale supérieure des mines de Paris École or Ecole may refer to: * an elementary school in the French educational stages normally followed by secondary education establishments (collège and lycée) * École (river), a tributary of the Seine flowing in région Île-de-France * Éco ...

, France and at the

Natural History Museum, London The Natural History Museum in London is a museum that exhibits a vast range of specimens from various segments of natural history. It is one of three major museums on Exhibition Road in South Kensington, the others being the Science Museum (Lo ...

, England.

References

{{Reflist Arsenate minerals Triclinic minerals Minerals in space group 2