Penroseite is a rare selenide mineral

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:* athabascaite Cu5Se4* clausthalite PbSe* penroseite Se2* stilleite ZnSe...

 with formula (Ni,Co,Cu)Se₂. It has a gray-steel color and black streak with a hardness of 3. It is an isometric

Cubic crystal system

In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....

 mineral, 2/m. Penroseite was first discovered in 1925 in a Bolivian rhyolite

Rhyolite

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...

. It was named for Richard Penrose

R. A. F. Penrose, Jr.

Richard Alexander Fullerton Penrose, Jr., better known throughout his career as R. A. F. Penrose, Jr., was an American mining geologist and entrepreneur....

 (1863-1931), an economic geologist

Economic geology

Economic geology is concerned with earth materials that can be used for economic and/or industrial purposes. These materials include precious and base metals, nonmetallic minerals, construction-grade stone, petroleum minerals, coal, and water. The term commonly refers to metallic mineral deposits...

.

Penroseite is a rare mineral found in the Pacajake mines in Bolivia

Bolivia

Bolivia officially known as Plurinational State of Bolivia , is a landlocked country in central South America. It is the poorest country in South America...

. 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

Pyrite

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

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

Penroseite makes extensive solid solution

Solid solution

A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...

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

Selenide

A selenide is a chemical compound in which selenium serves as an anion with oxidation number of −2 , much as sulfur does in a sulfide. The chemistry of the selenides and sulfides are similar....

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

Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...

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

Masuyite

Masuyite is a uranium/lead oxide mineral with formula Pb[3O32]·3H2O.Masuyite was first described in 1947 for an occurrence in Katanga and named to honor Belgian geologist Gustave Masuy ....

-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

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...

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 doesn’t show any activity(Gordon, 1926).

Geologic occurrence

The ore is found in the Pacajake mine in Colquechaca, 150 km southeast of Oruro, Bolivia. Penroseite comes from the vicinity, and forms in fissure veins in the Rhyolite and it is not found in-situ, in float instead. The ore is a bit float from undiscovered vein, which give uncertainty of the place that it has originally formed(Gordon, 1926).

Structure

Penroseite has a cubic space group Pa3(Bayliss, 1989). It was found that it behaves like isotropic crystals in which light pass throw 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 have the same structure as penroseite and thought to have similar coordination, has Fe atom surrounded by six atoms of S, octahedral coordination. On the other hand, S atom is surrounded by 4 Fe atoms in a tetrahedral configuration(Jaffe, 1988). This might give an idea of how the atoms arranged in the penroseite in the absence of a real structure determination due to the undefined composition of the mineral.