Penroseite is a rare

selenide mineral Selenide minerals are those minerals that have the selenide anion as a main component. Selenides are similar to sulfides and often grouped with them. Examples include: * achavalite * athabascaite * clausthalite * ferroselite * penroseite ...

with formula (Ni,Co,Cu)Se₂. It has a gray-steel color and black streak with a hardness of 3. It is an isometric mineral, 2/m. Penroseite was first discovered in 1925 in a Bolivian

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

. It was named for Richard Penrose (1863–1931), an economic geologist. Penroseite is a rare mineral found in the Pacajake mines in

Bolivia , image_flag = Bandera de Bolivia (Estado).svg , flag_alt = Horizontal tricolor (red, yellow, and green from top to bottom) with the coat of arms of Bolivia in the center , flag_alt2 = 7 × 7 square p ...

. It was discovered in 1925. It used to be found in fissure veins in the extrusive igneous rhyolite rock. It is considered as a member of the

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

group from the perspective of its structure, with a cubic space group (Bayliss, 1989). Penroseite makes extensive

solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...

s with other minerals. For example, penroseite can be a result of alteration process of many

selenide A selenide is a chemical compound containing a selenium anion with oxidation number of −2 (Se2−), much as sulfur does in a sulfide. The chemistry of the selenides and sulfides is similar. Similar to sulfide, in aqueous solution, the selenide ion ...

s, such as olsacherite Pb₂(SO₄)(SeO₄). Olsacherite forms very sparingly in well formed crystal covering the walls of the external side of the small cracks (Hurlbut, 1969). Another mineral related to penroseite is piretite. It occurs as an alteration product of

uraninite Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2 but because of oxidation typically contains variable proportions of U3O8. Radioactive decay of the uranium causes t ...

and primary selenium-bearing sulfides, such as penroseite. Piretite forms as crusts in association with an orange

masuyite Masuyite is a uranium/lead oxide mineral with formula Pb UO2)3O3(OH)2�3H2O. Masuyite was first described in 1947 for an occurrence in Katanga and named to honor Belgian geologist Gustave Masuy (1905–1945). See also * Classification of miner ...

-like U-Pb oxide on the surface of uraninite samples (Vochten, 1996).

Composition

Its composition have mixture of elements, some of them are primary elements such as nickel (Ni), copper (Cu), cobalt (Co), and selenium (Se) that make with each other a substitution in different amounts. The secondary elements, that might be found in penroseite due to the occurrence process or the environment in which the mineral was deposited, are silver (Ag), mercury (Hg), and lead (Pb)(Gordon, 1926). Lead, that was considered as a part of the composition when it was discovered, found to be in the composition due to the intergrowth of

clausthalite Clausthalite is a lead selenide mineral, PbSe. It forms a solid solution series with galena PbS. Occurrence It occurs in low-sulfur hydrothermal deposits with other selenides and in mercury deposits. It is associated with tiemannite, klockmann ...

PbSe and is not a primary element(Earley, 1950). Penroseite has a high content of selenium like no other mineral, and as a result it was reported as the first nickel selenide mineral discovered (Gordon, 1926).

Physical properties

Penroseite is a massive mineral. It can have radiating, columnar, or granular structure (Gordon, 1926). It shows a dull lead-gray to steel-gray color with a black streak. It has a metallic luster and thought to have a weak chemical bonding and that what makes it with a low hardness around 3 (Earley, 1950). When it reacts with HNO₃ or KCN, a fumes tarnish to brown are resulted. But if it reacts with HCl, FeCl₃, or HgCl₂, it does not show any activity (Gordon, 1926).

Geologic occurrence

The mineral is found in the Pacajake mine in Colquechaca, southeast of

Oruro Department Oruro (; Quechua: ''Uru Uru''; Aymara: ''Ururu'') is a department of Bolivia, with an area of . Its capital is the city of Oruro. According to the 2012 census, the Oruro department had a population of 494,178. Provinces of Oruro The departme ...

. Penroseite comes from the vicinity, and forms in fissure

veins Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...

. It is not found in-situ, rather only in float (loose rocks) from undiscovered veins which have either been eroded away or remain hidden in the mountain above (Gordon, 1926).

Structure

Penroseite has a cubic space group Pa3 (Bayliss, 1989). It was found that it behaves like isotropic crystals in which light passes through all directions with the same speed, which is strong enough to conclude that it is isometric (Bannister, 1937). Since it is isometric, all axes equal the same value. This value, due to different measurement techniques, ranges between 6.001 and 6.017 angstrom (Earley, 1950). Since there is no brief description on penroseite's structure, it almost have the same Pyrite subgroup structure (Earley, 1950). The type of bonds found in the Pyrite group is covalent, but not in penroseite. For example, pyrite FeS₂, which has the same structure as penroseite and is thought to have similar coordination, has one Fe atom surrounded by six atoms of S, an octahedral coordination. On the other hand, the S atom is surrounded by 4 Fe atoms in a tetrahedral configuration (Jaffe, 1988). This might give an idea of how the atoms are arranged in penroseite in the absence of a real structure determination due to the undefined composition of the mineral.

References

*Bannister, F. (1937) The identity of penroseite and Blockite. ''The American Mineralogist''. 22, 319–324. *Bayliss, P. (1989) Crystal chemistry and crystallography of some minerals within the pyrite group. ''The American Mineralogist''. 74, 1168–1176. *Earley, J. (1950) Description and synthesis of the selenide minerals. ''The American Mineralogist''. 35, 360–362. *Gordon, S. (1926) Penroseite and Trudellite: two new minerals. ''Proceedings Of The Academy Of Natural Sciences Of Philadelphia''. 77, 317–324. *Hurlbut, G. (1969) Olsacherite Pb(SO4)(SeO4), a new mineral from Bolivia. ''The American Mineralogist''. 54, 1519–1527. *Jaffe, H. (1988) Crystal Chemistry And Refractivity. 36–38. *Stanly-Brown, J. (1932) Memorial of Richard Alexander Fullerton Penrose. ''Bulletin of the Geological Society of America''. 43, 68–104. *Vochten, R., Blaton, N., Peeters, O., Deliens, M. (1996) Piretite, Ca(UO2)3(SeO3)2(OH)4.4H2O, a new calcium uranyl selenite from Shinkolobwe, Shaba, Zaire. ''Canadian Mineralogist''. 34, 1317–1322. {{Refend Nickel minerals Cobalt minerals Copper minerals Pyrite group Selenide minerals Cubic minerals Minerals in space group 205