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 maficdiatreme 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 P1space group and to 1 or 1point 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]
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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.
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Evans, J.R.; Allmann, R. (1968). The crystal structure and crystal chemistry of Valleriite. Vol. 127. Zeitschrift flir Kristallographie, Bd. pp. 73–93.
^ abc
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)