Talmessite is a

hydrated Drinking is the act of ingesting water or other liquids into the body through the mouth, proboscis, or elsewhere. Humans drink by swallowing, completed by peristalsis in the esophagus. The physiological processes of drinking vary widely among ot ...

calcium Calcium is a chemical element with the 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 ...

magnesium Magnesium is a chemical element with the 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 of the periodic ta ...

arsenate The arsenate ion is . An arsenate (compound) is any compound that contains this ion. Arsenates are salts or esters of arsenic acid. The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V). Arsenate resem ...

, often with significant amounts of

cobalt Cobalt is a chemical element with the 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. The free element, pr ...

nickel Nickel is a chemical element with 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 slow to ...

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

Anarak Anarak ( fa, انارک, also Romanized as Anārak) is a city and capital of Anarak District, in Nain County, Isfahan Province, Iran. At the 2006 census, its population was 1,285, in 462 families. It is situated at an altitude of . Anarak is lo ...

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 with the symbol Zn and atomic number 30. Zinc 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 periodi ...

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 * 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 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 is ...

mineral,

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 u ...

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 ...

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, for example) does not necessaril ...

(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, 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 ...

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

tetrahedral 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 ...

AsO₄ and

octahedral 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 at ea ...

cation-O₄(H₂O)₂)groups that parallel 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 ...

. The octahedral are compressed, resulting in chain disorder

Appearance

Talmessite occurs as

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 ...

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 Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is th ...

, 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 (British 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 generally imp ...

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 various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

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 sca ...

but not as hard as

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 ...

. 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 betwee ...

and the

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 w ...

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 consid ...

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 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 is ...

crystals (and

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

and

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 ...

crystals) have two directions in which light travels with zero

birefringence Birefringence 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 said to be birefringent (or birefractive). The birefring ...

; 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, which is called the coefficient of proportionality or proportionality constan ...

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 Acute may refer to: Science and technology * Acute angle ** Acute triangle ** Acute, a leaf shape in the glossary of leaf morphology * Acute (medicine), a disease that it is of short duration and of recent onset. ** Acute toxicity, the adverse eff ...

, 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, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

, and hence with

colour 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 associ ...

. 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 In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to ...

). 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 (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 ...

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 primary rock and includes the essential mineral(s) used to classify the rock along with any accessory minerals. In ore deposit geology, hypogene processe ...

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 "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, often in assoc ...

orpiment Orpiment 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 is formed both by sublimation and as a byproduct of the decay of another a ...

, 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 stoichiometries, and in this respect ars ...

Cobalt Cobalt is a chemical element with the 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. The free element, pr ...

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

erythrite Erythrite or red cobalt is a secondary hydrated cobalt arsenate mineral with the formula . Erythrite and annabergite, chemical formula , or nickel arsenate form a complete series with the general formula . Erythrite crystallizes in the monoclin ...

annabergite Annabergite is an arsenate mineral consisting of a hydrous nickel arsenate, Ni3(AsO4)2·8H2O, crystallizing in the monoclinic system and isomorphous with vivianite and erythrite. Crystals are minute and capillary and rarely met with, the mineral ...

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 gypsum ...

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 tho ...

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, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...

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 Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

. At the type locality it is associated with

and

Localities

The type locality is the Talmessi Mine,

District, Nain County, Esfahan Province, Iran, and

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

is conserved at the

École nationale supérieure des mines de Paris Mines Paris - PSL, officially École nationale supérieure des mines de Paris (until May 2022 Mines ParisTech, also known as École des mines de Paris, ENSMP, Mines de Paris, les Mines, or Paris School of Mines), is a French grande école and a c ...

, 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 an ...

, England.

References

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