Murdochite
| Murdochite | |
|---|---|
Murdochite
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| General | |
| Category | Halide minerals |
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Formula (repeating unit) |
PbCu6O8−x(Cl,Br)2x(x ≤ 0.5) |
| Strunz classification | 3.DB.45 |
| Crystal system | Cubic |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m) |
| Space group | Fm3m |
| Identification | |
| References | |
Murdochite is a mineral combining lead and copper oxides with formula PbCu6O8−x(Cl,Br)2x(x ≤ 0.5).
It was first discovered in 1953 in the Mammoth-Saint Anthony Mine in Pinal County, Arizona by Percy W. Porter, a mining engineer who handpicked a 401.5-mg sample. Porter would later submit for analysis and it was then that Fred A. Hildebrand suggested that the sample was a new mineral after taking a powder x-ray picture. It was named for Joseph Murdoch (1890–1973), American mineralogist. Murdochite was first suggested to be of a cubic structure. After this suggestion, the term “murdochite-type structure” began to be used when describing a structure that is similar to that of murdochite. Murdochite was later found to be octahedral.
Various studies have examined the composition of murdochite. When murdochite was first discovered, its chemical formula was determined through stoichiometry to be Cu6•1Pb0•9O8•O. The ideal formula for murdochite was originally thought to be Cu6PbO8. Unfortunately, this formula failed to take into consideration the significant amounts of chlorine and bromine present in murdochite. Using electron microprobe analysis, a new chemical composition for murdochite was determined that included chlorine and bromine. This new formula was Cu6+xPb+x(O,Cl,Br)8 and was created with regard to the Cu/Pb variance observed in samples of murdochite from both Hansonburg, New Mexico, and T. Khuni mine, Iran. Again this new composition was proven incorrect and the formula that is widely accepted today, PbCu6O8−x(Cl,Br)2x(x ≤ 0.5), would be proposed.
The structure originally proposed for murdochite was a simple cubic structure. This structure was later proven incorrect and determined to be octahedral. The structure of murdochite is described as edges of [PbO8] cubes spanned by Cu2+ ions that give a square-planar CuO4 arrangement in a three-dimensional network.Halogen ions complete a (4 + 2_ elongated CuO4(Cl, Br)2 octahedron.
Murdochite is an opaque mineral that exhibits a black color, adamantine luster, black streak, and isotropic optical class. Samples from T. Khuni mine indicate that polished sections of murdochite resemble the color and reflectivity of magnetite but vary because of zoning. The lighter zones in murdochite are caused because they are richer in lead than the darker zones. The mineral is known to be brittle, have a hardness of 4, and have {111} cleavage. Murdochite also has a pink tinge that is distinctive and its edges seem to be more reflective than the center because samples are commonly porous in the center and compact on the sides with clean-cut faces. The space group of murdochite has been confirmed to be Fm3m. Murdochite can be cubic, with a point group of 4/m 3 2/m, or octahedral and cubo-octahedral whose point group is 2 mm. Murdochite can occur in the forms (100) and (111) with crystal twinning being very common. The calculated density of murdochite was determined to be 6.1 g/cm3 and have a measured density of 6.47 g/cm3.
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