Pearceite is one of the four so-called "ruby silvers", pearceite , pyrargyrite , proustite and

miargyrite Miargyrite is a mineral, a sulfide of silver and antimony with the formula Ag Sb S2. It is a dimorph of cuboargyrite. Originally discovered in the Freiberg district of Germany in 1824, it has subsequently been found in many places where silver ...

. It was discovered in 1896 and named after Dr Richard Pearce (1837–1927), a Cornish–American chemist and metallurgist from

Denver Denver () is a consolidated city and county, the capital, and most populous city of the U.S. state of Colorado. Its population was 715,522 at the 2020 census, a 19.22% increase since 2010. It is the 19th-most populous city in the Unit ...

, Colorado.

Nomenclature

Pearceite and polybasite are closely related minerals that form the pearceite-polybasite series. Originally pearceite was thought to be an

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, b ...

analogue of polybasite , and was called arsenpolybasite, and one

polytype In materials science, polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism. Any crystalline material can exhibit the phenomenon. Allotropy refers to polymorphism ...

of polybasite was called antimonpearceite. Arsenpolybasite was found to represent two different polytypes, arsenpolybasite-221 and arsenpolybasite-222.Frondel, C (1963) Isodimorphism of the polybasite and pearceite series. American Mineralogist 48:565–572 In modern usage the old name pearceite is replaced by the polytype name pearceite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c and antimonpearcite by polybasite-Tac.Bindi L, Evain M, Spry P G and Menchetti S (2007) The pearceite-polybasite group of minerals: crystal chemistry and new nomenclature rules, American Mineralogist 92:918-925 Pearceite-Tac forms a series with polybasite-Tac.

Crystallography and Structure

Two structural varieties,

trigonal In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crystal ...

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 ...

, are known. The trigonal variety crystallizes in the hexagonal scalenohedral class m ( 2/m),

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 ...

Pm1 (P 2/m 1). The monoclinic variety crystallises in the

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 ...

2/m class, space group C2/m.

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 *Monoclinic variety: There are two formula units per unit cell (Z = 2), the lengths of the sides of the unit cell are a = 12.64 Å, b = 7.29 Å, c = 11.90 Å and the angle between the c and a directions is β = 90.0°. *Trigonal variety: There is one formula unit per unit cell (Z = 1), two of the sides are of equal length a = 7.3876 Å and the third side, parallel to the threefold axis, is c = 11.8882 Å. The crystal structure consists of sheets stacked along the c axis. The

atoms form isolated pyramids,

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...

cations 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 con ...

link two

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

atoms and the

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

cations are found in various sites with low

coordination number In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central io ...

s, 2,3 and 4, as is usually the case with silver.Bindi L, Evain M, Menchetti S Acta Crystallographica B62 (2006) 212-219 Temperature dependence of the silver distribution in the crystal structure of natural pearcite,

Properties

Pearceite is often granular and massive; crystals are short, tabular pseudo

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

prisms Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentar ...

with bevelled edges, showing triangular striations on faces parallel to the plane containing the a and b

axes Axes, plural of '' axe'' and of '' axis'', may refer to * ''Axes'' (album), a 2005 rock album by the British band Electrelane * a possibly still empty plot (graphics) See also * Axess (disambiguation) *Axxess (disambiguation) Axxess may refer to ...

, and rosettes of such crystals, to 3 cm across. The mineral is black, and in polished section it is white with very dark red internal reflections. It has a black to reddish black streak and a metallic luster, generally opaque, but translucent in very thin fragments. It is biaxial with a very high

refractive index 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, ...

of 2.7 and maximum

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 birefri ...

δ also 2.7.

Dispersion Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...

of the optic axes is relatively strong. Reflected light anisotropism is the property of appearing to change color when viewed under crossed polarised light in a reflected light microscope. Pearceite exhibits moderate anisotropism, often dark violet. The color in reflected plane polarised light is white, with very dark red internal reflections and very weak pleochroism in air, fair in oil.

Reflectivity The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...

in air at 540 nm is about 30%. It is not

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, ...

. Pearceite is a brittle mineral that breaks with a

conchoidal Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically slig ...

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 displ ...

. It is soft, with

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

only 3, the same as

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

. The silver content gives it a high

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 at its dens ...

of 6.15, the highest of the ruby silvers. Cleavage is either absent or poor. The mineral is neither

magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particl ...

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 consi ...

Occurrence and associations

The type locality is the Mollie Gibson Mine,

Aspen Aspen is a common name for certain tree species; some, but not all, are classified by botanists in the section ''Populus'', of the '' Populus'' genus. Species These species are called aspens: *'' Populus adenopoda'' – Chinese aspen (C ...

, Aspen District (Roaring Fork District),

Pitkin County, Colorado Pitkin County is a county in the U.S. state of Colorado. As of the 2020 census, the population was 17,358. The county seat and largest city is Aspen. The county is named for Colorado Governor Frederick Walker Pitkin. Pitkin County has the sev ...

, where the mineral occurs in hydrothermal deposits formed at low to medium temperatures, associated with

acanthite Acanthite is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below . Argentite is the stable form above that temperature. As argentite cools below that te ...

tetrahedrite Tetrahedrite is a copper antimony sulfosalt mineral with formula: . It is the antimony endmember of the continuous solid solution series with arsenic-bearing tennantite. Pure endmembers of the series are seldom if ever seen in nature. Of the t ...

, native silver, proustite,

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 f ...

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), ...

and

. Type material is lodged at

Yale University Yale University is a private research university in New Haven, Connecticut. Established in 1701 as the Collegiate School, it is the third-oldest institution of higher education in the United States and among the most prestigious in the w ...

, New Haven, Connecticut, references 3.4270, 3.4292, 3.4293, and at The

Natural History Museum A natural history museum or museum of natural history is a scientific institution with natural history collections that include current and historical records of animals, plants, fungi, ecosystems, geology, paleontology, climatology, and more. ...

, London, England, reference 84843.

References

External links

Nikon: Introduction to Polarized Light Microscopy

{{Commons category Sulfide minerals Silver minerals Copper(I) minerals Arsenic minerals Aluminium minerals Trigonal minerals Minerals in space group 164 Monoclinic minerals