Coyoteite
General
CategorySulfide mineral
Formula
(repeating unit)		(NaFe3S5·2H2O)
IMA symbolCoy[1]
Strunz classification2.FD.25
Dana classification02.14.06.01
Crystal systemTriclinic
Unknown space group
Identification
Colorblack
Crystal habitIrregular grains - occurs as splotchy, anhedral crystals forming inclusions in other minerals
Cleavage{111} Perfect
Mohs scale hardness1-1.5
LusterMetallic
Streakblack
DiaphaneityOpaque
Specific gravity2.5 - 2.62 measured; 2.879 calculated
PleochroismFaint, from gray to pink
Other characteristicsModerately magnetic
References[2][3][4][5]

Coyoteite is a hydrated sodium iron sulfide mineral. The mineral was named coyoteite after Coyote Peak near Orick, California, where it was discovered (along with another rare mineral, orickite).[2]

This mineral is unstable under normal atmospheric conditions, making it rare at the surface. The mineral was first described in a petrographic study of a sample of a mafic diatreme at Coyote Peak. The largest piece of coyoteite found on that specimen has the dimensions of 0.2 × 0.4 mm.[2]

Chemical composition and properties

The chemical formula of the mineral is NaFe3S5·2H2O.[2] It was obtained by the analysis of five grains using the 10-kV EMX-SM electron microprobe.[2] 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 and oxygen, which are undetectable by the electron microprobe.[2] It was unclear however whether the hydrogen and oxygen are in the form of water or a hydroxide.[2]

The mineral is insoluble in cold acids, such as HCl, but dissolves upon heating.[2]

Physical and optical properties

Coyoteite is black in color and is streaked with opaque metallic luster in fragmental samples.[2] The mineral has perfect cleavage on the {111} planes and shows a unique chevron pattern because of the diametrical arrangement of these crystallographically equivalent cleavage planes.[2] The measured density 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 between the cleavage planes.[2] The hardness is 1.5 on Mohs scale and is comparable to that of erdite (NaFeS2·2H2O) which has a similar chemical composition. Coyoteite is moderately magnetic.[2] On the polished thin section, coyoteite is pale brownish to gray, with pink tint. It shows low pleochroism, from gray to pink. In reverse, it shows large optical anisotropy, from gray to dull golden orange.[2]

Crystallography

The X-ray precession image classifies the coyoteite crystal system as triclinic. The crystal belongs to P1 or P1 space group and to 1 or 1 point group. Imaging shows that coyoteite has poor crystal quality.[2] 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 volume is V = 446.2 Å3.[2]

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, 4(Fe,Cu)S•3(Mg,Al)(OH)
2. Valleriite is a layered mineral, it consists of layers of (Mg0.68Al0.32(OH)2) and layers of (Fe0.07Cu0.93S2).[4]

Geologic occurrence

Coyoteite has been found only in the mafic alkalic diatreme at the Coyote Peak near Orick, Humboldt County, California, US.[2][5] It is formed in pegmatitic clots associated with some rare iron sulfide minerals, such as erdite, bartonite and djerfisherite, as well as with common minerals pyrrhotite and magnetite.[5]

References

  1. ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Erd, Richard C.; Czamanske, Gerald K. (1983). "Orickite and coyoteite, two new sulfide minerals from Coyote Peak, HunboldtCountry, California" (PDF). American Mineralogist. 68: 245–254.
  3. ↑ V. Pekov, A.A. Agakhanov (2007). Thallium-Rich Murunskite from the Lovozero Pluton, Kola Peninsula, and Partitioning of Alkali Metals and Thallium between Sulfide Minerals. Vol. 4. Zapiski Rossiskogo Mineralogicheskogo Obshchestva. pp. 74–78.
  4. 1 2 Evans, J.R.; Allmann, R. (1968). The crystal structure and crystal chemistry of Valleriite. Vol. 127. Zeitschrift flir Kristallographie, Bd. pp. 73–93.
  5. 1 2 3 Anthony, J. W., Bedeaux, R., Bladh, K. and Nicholas, M. (1990). Handbook of Mineralogy (PDF). Vol. 1. Mineral Data Publishing (Republished by the Mineralogical Society of America).{{cite book}}: CS1 maint: multiple names: authors list (link)
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