Carminite (PbFe³⁺₂(AsO₄)₂(OH)₂) is an anhydrous

arsenate mineral Arsenate minerals usually refer to the naturally occurring orthoarsenates, possessing the (AsO4)3− anion group and, more rarely, other arsenates with anions like AsO3(OH)2− (also written HAsO42−) (example: pharmacolite Ca(AsO3OH).2H2O) or ( ...

containing

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 ...

. It 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 ...

that is structurally related to palermoite (Li₂SrAl₄(PO₄)₄(OH)₄). Sewardite (CaFe³⁺₂(AsO₄)₂(OH)₂) is an analogue of carminite, with

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 t ...

in sewardite in place of the

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

in carminite. Mawbyite is a dimorph (same formula, different structure) of carminite; mawbyite is

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 ...

and carminite is orthorhombic. It has a

molar mass In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecular, ...

of 639.87 g. It was discovered in 1850 and named for the characteristic carmine colour.

Structure

Carminite belongs to the orthorhombic crystal class (2/m 2/m 2/m) and has

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 ...

C ccm or C cc2.Finney J J (1963) American Mineralogist 48: 1-13 The structure consists of linked

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 ...

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

surrounded by

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 ...

and

which are aligned parallel to the c axis. They are connected together in the direction of the a axis by

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 res ...

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 ...

(

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, ...

surrounded by 4

). Coordination about the

atoms is eight-fold. The edges of the

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 ...

have lengths a = 16.59 Å, b = 7.58 Å and c = 12.295 Å. There are 8 formula units in each unit cell (Z = 8).

Appearance

Crystals have been found up to 2 cm long, though most are smaller. They are typically bladed, elongated along the c axis and flattened perpendicular to the b axis. They also occur as acicular crystals, in

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

or tufted aggregates and as

fibrous Fiber or fibre (from la, fibra, links=no) is a #Natural fibers, natural or Fiber#Artificial fibers, artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The stronge ...

drusy In geology, druse refers to a coating of fine crystals on a rock fracture surface or vein or within a vug or geode. See also * Crystal habit In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crys ...

masses. The crystals are a characteristic carmine red colour, hence the name, and they are also red in transmitted light. They are

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions ...

with a vitreous lustre and a reddish yellow streak.

Physical properties

Carminite is fairly soft, with a

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 t ...

of , between that of calcite and

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 ...

. Because of the

content it is heavy, with

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 densest ...

of 5.03 - 5.18, although specimens from Mapimi are less dense at 4.10. Cleavage is distinct in one direction parallel to the c axis. The mineral is slowly soluble in

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

(HCl) with the separation of

lead(II) chloride Lead(II) chloride (PbCl2) is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead(II) chloride is one of the most important lead-based reagents. It also occurs naturally in the form of the mi ...

(PbCl₂) and totally soluble in

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

(HNO₃). Carminite 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 consi ...

and no

piezoelectric effect Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word '' ...

has been detected.

Optical properties

Orthorhombic crystals (and monoclinic and

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 ...

crystals) have two directions in which light travels with zero birefringence; these directions are called the optic axes, and the crystal is said to be 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 In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It ca ...

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 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, or ...

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 Carminite is orthorhombic and for an orthorhombic crystal the optical directions correspond to the crystal axes a, b and c, but not necessarily in that order. For carminite the orientation is X = c, Y = a and Z = b and the

refractive indices 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, or ...

are high, with n_α = 2.070, n_β = 2.070, n_γ = 2.080, only a little less than

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

at 2.4.Foshag (1937) American Mineralogist 22: 479-484 The maximum birefringence δ is the difference between the highest and lowest refractive index; for carminite δ = 0.010.
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. Carminite is biaxial (+) and 2V is moderate to large. 2V depends on the

, 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, t ...

, and hence with colour. 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). If 2V is greater for red light than for violet light the dispersion is designated r > v, and vice versa. For carminite the dispersion is strong, with r < v.
The mineral exhibits strong

pleochroism 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. Backgrou ...

; when viewed along the X direction it appears pale yellowish red and dark carmine red along the Y and Z directions. Absorption is equal along the Y and Z optic directions, but less along the X optic direction.
When a birefringent crystal is rotated between crossed polarizers it will turn dark every 90° of rotation. This effect is known as

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

. Carminite exhibits the parallel extinction that is characteristic of orthorhombic crystals.

Occurrence

Carminite is formed as an uncommon alteration product of

arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elem ...

(FeAsS) in the oxidized zones of some lead-bearing deposits. Common associates are

wulfenite Wulfenite is a lead molybdate mineral with the formula Pb Mo O4. It can be most often found as thin tabular crystals with a bright orange-red to yellow-orange color, sometimes brown, although the color can be highly variable. In its yellow form ...

scorodite Scorodite is a common hydrated iron arsenate mineral, with the chemical formula FeAsO4·2H2O. It is found in hydrothermal deposits and as a secondary mineral in gossans worldwide. Scorodite weathers to limonite. Scorodite was discovered in the ...

, plumbojarosite,

mimetite Mimetite is a lead arsenate chloride mineral (Pb5(AsO4)3Cl) which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. The name derives from the Greek Μιμητής ''mimetes'', meaning "imitator" a ...

, dussertite,

cerussite Cerussite (also known as lead carbonate or white lead ore) is a mineral consisting of lead carbonate (PbCO3), and is an important ore of lead. The name is from the Latin ''cerussa'', white lead. ''Cerussa nativa'' was mentioned by Conrad Gessner ...

beudantite Beudandite is a secondary mineral occurring in the oxidized zones of polymetallic deposits. It is a lead, iron, arsenate, sulfate with endmember formula: PbFe3(OH)6SO4AsO4. Beudantite is in a subgroup of the alunite group. It is the arsenate an ...

bayldonite Bayldonite (BAIL-done-ite) is a rare secondary mineral with the chemical formula PbCu3(AsO4)2(OH)2. It was first discovered in Penberthy Croft Mine, Cornwall, England, United Kingdom. It is named after its discoverer, John Bayldon (1837(8) – 187 ...

arseniosiderite Arseniosiderite is a rare arsenate mineral formed by the oxidation of other arsenic-containing minerals, such as scorodite or arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metal ...

and

anglesite Anglesite is a lead sulfate mineral with the chemical formula PbSO4. It occurs as an oxidation product of primary lead sulfide ore, galena. Anglesite occurs as prismatic orthorhombic crystals and earthy masses, and is isomorphous with barite and ...

. The type locality is the Louise Mine,

Bürdenbach Bürdenbach is a municipality in the district of Altenkirchen, in Rhineland-Palatinate, Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe a ...

, Altenkirchen, Wied Iron Spar District, Westerwald,

Rhineland-Palatinate Rhineland-Palatinate ( , ; german: link=no, Rheinland-Pfalz ; lb, Rheinland-Pfalz ; pfl, Rhoilond-Palz) is a western state of Germany. It covers and has about 4.05 million residents. It is the ninth largest and sixth most populous of the ...

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

where it is associated with

. At the

Hingston Down Hingston Down is a hill not far from Gunnislake in Cornwall, England, United Kingdom. It is the subject of an old rhyme, due to the prolific tin mining that formerly took place in the area: This Hingston Down should not be confused with the ...

Consols mine in

Cornwall Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

, carminite occurs with

and

pharmacosiderite Pharmacosiderite is a hydrated basic ferric arsenate, with chemical formula K Fe4( As O4)3(O H)4·(6-7) H2O and a molecular weight of 873.38 g/mol. It consists of the elements arsenic, iron, hydrogen, potassium, sodium and oxygen. It has a M ...

. The ores of the Ojuela Mine,

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

, are replacement deposits in

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and consist of galena, sphalerite,

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

, and

in a

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...

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 ...

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 ...

and

. Arsenopyrite is abundant. On a dump near the north shaft blocks of massive

containing seams and pockets of

and small areas of dussertite and carminite have been found. Carminite also occurs as masses mixed with

and plumbojarosite. It is almost always intimately associated with

and dussertite.

References

{{Reflist Arsenate minerals Iron(III) minerals Lead minerals Orthorhombic minerals Minerals in space group 66