Murdochite is a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

combining

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 ...

and

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

oxides with the chemical formula (x ≤ 0.5). It was first discovered in 1953 in the Mammoth-Saint Anthony Mine in

Pinal County, Arizona Pinal County is in the central part of the U.S. state of Arizona. According to the 2020 census, the population of the county was 425,264, making it Arizona's third-most populous county. The county seat is Florence. The county was founded in 187 ...

by Percy W. Porter, a mining engineer who handpicked a 401.5-mg sample.Fahey, Josepf J., (1955) Murdochite, A new Copper Lead Oxide Mineral. American Mineralogist 40, 905-906. 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.http://www.mindat.org/min-2812.html Mindat.org Murdochite was first suggested to be of a cubic structure.Christ, C. L., and Clark, Joan R. (1955) The Crystal Structure of Murdochite. American Mineralogist 40, 907-916. After this suggestion, the term “murdochite-type structure” began to be used when describing a structure that is similar to that of murdochite.Winkler, Björn, Chall, Michael, Pickard, Chris J., Milman, Victor, and White, Jim. (2000)Structure of Cu₆PbO₈. Acta Crystallographica 56, 22-26. Murdochite was later found to be octahedral.Dubler, Erich, Vadani, Angelo, and Oswald, Rudolf Hans. (1983) New Structure Determination of Murdochite, Cu₆PbO₈. Acta Crystallographica 39, 1143-1146.

Composition

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 . This formula failed to take into consideration the significant amounts of

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

and

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...

present in murdochite. Using

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 ...

analysis, a new chemical composition for murdochite was determined that included chlorine and bromine. This new formula was 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.Burke, E. A. J., and Maaskant, P. (1970) New Data on Murdochite. Nues Jahrb. Mineral Montlsh. 558-565. ). This new composition was also proven incorrect and the formula that is widely accepted today, (x ≤ 0.5), was proposed.

Structure

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 bO₈cubes spanned by Cu²⁺ ions that give a square-planar CuO₄ arrangement in a three-dimensional network. Halogen ions complete a (4 + 2_ elongated octahedron.

Physical properties

Murdochite is an

opaque Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** In lingu ...

mineral that exhibits a black color, adamantine luster, black streak, and isotropic optical class.Anthony, J.W., Bideaux, R., Bladh, K. and Nichols, M. (2003) Murdochite (x < 0.5). Handbook of Mineralogy. Mineral Data Publishing (Republished by the Mineralogical Society of America). Samples from T. Khuni mine indicate that polished sections of murdochite resemble the color and reflectivity of

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 th ...

but vary because of zoning.Adib, D., and Ottermann, J. (1970) Some New Lead Oxide Minerals and Murdochite from T. Khuni Mine, Anarak, Iran. Mineralium Deposita 5, 1, 86-93. The lighter zones in murdochite are caused because they are richer in

than the darker zones. The mineral is known to be brittle, have a hardness of 4, and have 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 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 ...

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 Crystal twinning occurs when two or more adjacent crystals of the same mineral are oriented so that they share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals that are tightly ...

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.

Geologic occurrence

Naturally occurring murdochite, (x ≤ 0.5), was discovered in Mammoth Mine, Arizona. The mine, about 46 miles northeast of

Tucson , "(at the) base of the black ill , nicknames = "The Old Pueblo", "Optics Valley", "America's biggest small town" , image_map = , mapsize = 260px , map_caption = Interactive map ...

, contains deposits of molybdenum,

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

, vanadium, sphalerite, and galena. In this mine, murdochite was found embedded in plates of

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 ...

as well as on the surface of crystals of

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 ...

. Other crystals found in Mammoth Mine include

hemimorphite Hemimorphite is the chemical compound Zn4( Si2O7)( OH)2 ·H2O, a component of mineral calamine. It is a silicate mineral which, together with smithsonite (ZnCO3), has been historically mined from the upper parts of zinc and lead ores. Both ...

willemite Willemite is a zinc silicate mineral () and a minor ore of zinc. It is highly fluorescent (green) under shortwave ultraviolet light. It occurs in a variety of colors in daylight, in fibrous masses and apple-green gemmy masses. Troostite is a vari ...

, and

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 ...

. Larger masses of murdochite have also been found at the T. Kguni Mine in Anarak, Iran. Here murdochite was found in conjunction with khuniite (

iranite Iranite ( Persian: ایرانیت) is a triclinic lead copper chromate silicate mineral with formula Pb10Cu(CrO4)6(SiO4)2(F,OH)2. It was first described from an occurrence in Iran. It is the copper analogue of hemihedrite (Pb10Zn(CrO4)6(SiO4)2 ...

), chrominium (

phoenicochroite Phoenicochroite, also known as melanochroite, is a lead chromate mineral with formula Pb2OCrO4. It forms striking orange red crystals. It was first discovered in 1839 in Beryozovskoye deposit, Urals, Russia Russia (, , ), or the Russia ...

), plumangite and an unknown mineral with the formula Pb₉O₁₆. All these minerals are lead oxide minerals, meaning that murdochite can be found in oxidized Pb-Cu deposits. More recently, small murdochite crystals were found encrusting

aurichalcite Aurichalcite is a carbonate mineral, usually found as a secondary mineral in copper and zinc deposits. Its chemical formula is . The zinc to copper ratio is about 5:4. Occurrence Aurichalcite typically occurs in the oxidized zone of copper and z ...

crystals in the Granite Gap Mines in New Mexico.Beyer, Joan. (2002) Minerals of the Granite Gap Mines. Rocks & Minerals 77, 5, 298-305.

Origin of the name

The mineral murdochite is named after Joseph Murdoch, who was once a professor of geology at the

University of California, Los Angeles The University of California, Los Angeles (UCLA) is a public land-grant research university in Los Angeles, California. UCLA's academic roots were established in 1881 as a teachers college then known as the southern branch of the California S ...

and a past president of the

Mineralogical Society of America The Mineralogical Society of America (MSA) is a scientific membership organization. MSA was founded in 1919 for the advancement of mineralogy, crystallography, geochemistry, and petrology, and promotion of their uses in other sciences, industr ...

.Webb, Robert W. (1975) Memorial of Joseph Murdoch February 19, 1890-December 31, 1973. American Mineralogist 60, 511-513. Professor Murdoch was born February 19, 1890, and died December 31, 1973, at 83 years of age. Murdoch obtained a baccalaureate degree, an M.S. and a Ph.D. in geologic science by 1915 from

Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of high ...

and would first be involved in a non-geological business career before joining the faculty at the department of geology at the University of California, Los Angeles in 1928. It was here that Murdoch returned to work in the geologic field that would lead to the publishing of numerous papers which would include 23 publications ranging from the years 1913–1968, along with ten published abstracts and many book reviews.

References