Coyoteite is a hydrated

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

sulfide mineral The sulfide minerals are a class of minerals containing sulfide (S2−) or disulfide (S22−) as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the ...

. The mineral was named coyoteite after Coyote Peak near

Orick, California Orick (formerly, Arekw, Orekw, and Oreq, Yurok: '' 'O'rekw '') is a census-designated place situated on the banks of the Redwood Creek in Humboldt County, California. It is located north of Eureka, at an elevation of 26 feet (8 m). The populat ...

, where it was discovered (along with another rare mineral, orickite). This mineral is unstable under normal atmospheric conditions, making it rare at the surface. The mineral was first described in a

petrographic Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The class ...

study of a sample of a

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

diatreme A diatreme, sometimes known as a maar-diatreme volcano, is a volcanic pipe formed by a gaseous explosion. When magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the ...

at Coyote Peak. The largest piece of coyoteite found on that specimen has the dimensions of 0.2 × 0.4 mm.

Chemical composition and properties

The chemical formula of the mineral is NaFe₃S₅·2H₂O. It was obtained by the analysis of five grains using the 10-kV EMX-SM

electron microprobe An electron microprobe (EMP), also known as an electron probe microanalyzer (EPMA) or electron micro probe analyzer (EMPA), is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It ...

. The total concentration of detected elements was less than 100% (90.7 weight percent). Qualitative analyses and TAP crystal were used to find the missing 9.31%. The results confirmed the missing elements to be

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

and

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 wel ...

, which are undetectable by the electron microprobe. It was unclear however whether the hydrogen and oxygen are in the form of water or a hydroxide. The mineral is insoluble in cold acids, such as HCl, but dissolves upon heating.

Physical and optical properties

Coyoteite is black in color and is streaked with opaque metallic luster in fragmental samples. The mineral has perfect cleavage on the planes and shows a unique

chevron Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * ''Eulithis testata'', a moth * Chevron (geology), a fold in rock lay ...

pattern because of the diametrical arrangement of these crystallographically equivalent cleavage planes. The measured

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

is 2.5 to 2.6 g/cm³ and the calculated one is 2.879 g/cm³. This significant density difference is because of the presence of

epoxy Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also coll ...

between the cleavage planes. The

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 ...

is 1.5 on

Mohs scale 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 ...

and is comparable to that of erdite (NaFeS₂·2H₂O) which has a similar chemical composition. Coyoteite is moderately

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 particle ...

. On the polished thin section, coyoteite is pale brownish to gray, with pink tint. It shows low

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. Backgroun ...

, from gray to pink. In reverse, it shows large

optical anisotropy 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 ...

, from gray to dull golden orange.

Crystallography

The X-ray precession image classifies the coyoteite

crystal system In crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices. Space groups are classified into crystal systems according to their po ...

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 ...

. The crystal belongs to P1 or P

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 ...

and to 1 or

point group In geometry, a point group is a mathematical group of symmetry operations (isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every p ...

. Imaging shows that coyoteite has poor crystal quality. The X-ray diffraction pattern suggests the following value for the crystallographic axes a = 7.409 Å, b = 9.881 Å, c = 6.441 Å; α = 100° 25’, β = 104° 37’ and γ = 81° 29’; 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 ...

volume is V = 446.2 Å³.

Possible atomic structure

Scarcity of coyoteite hinders attempts to determine its atomic structure. It is unknown whether the oxygen and the hydrogen are present in the mineral as H₂O molecules or OH⁻ anions. However, because of the softness and the lamellar cleavage in the mineral, coyoteite structure might be close to that of

valleriite Valleriite is an uncommon sulfide mineral (hydroxysulfide) of iron and copper with formula: or . It is an opaque, soft, bronze-yellow to brown mineral which occurs as nodules or encrustations. Discovery and occurrence Valleriite was first describ ...

, . Valleriite is a layered mineral, it consists of layers of (Mg_0.68Al_0.32(OH)₂) and layers of (Fe_0.07Cu_0.93S₂).

Geologic occurrence

Coyoteite has been found only in the mafic alkalic

at the Coyote Peak near Orick,

Humboldt County, California Humboldt County () is a county located in the U.S. state of California. As of the 2020 census, the population was 136,463. The county seat is Eureka. Humboldt County comprises the Eureka–Arcata–Fortuna, California Micropolitan Statistica ...

, US. It is formed in

pegmatitic A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...

clots associated with some rare iron sulfide minerals, such as erdite, bartonite and

djerfisherite Djerfisherite is an alkali copper–iron sulfide mineral and a member of the djerfisherite group. The chemical composition is somewhat variable. A Russian study from 1979 on djerfisherite from the Kola Peninsula found the formula , but a study in ...

, as well as with common minerals

pyrrhotite Pyrrhotite is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.2). It is a nonstoichiometric variant of FeS, the mineral known as troilite. Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is ...

and

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...

References

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External links

webmineral coyoteitehandbook of minerlogy coyoteite
Sodium minerals Iron(III) minerals Sulfide minerals Triclinic minerals