Penroseite
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| Penroseite | |
|---|---|
 Penroseite from Pakajake Canyon, Chayanta Province, Potosí Department, Bolivia | |
| General | |
| Category | Selenide minerals, pyrite group |
| Formula | (Ni,Co,Cu)Se2 |
| IMA symbol | Pen[1] |
| Strunz classification | 2.EB.05a |
| Dana classification | 02.12.01.04 |
| Crystal system | isometric |
| Crystal class | Diploidal (m3) H-M symbol: (2/m 3) |
| Space group | Pa3 |
| Identification | |
| Color | Steel gray |
| Crystal habit | Radial, columnar, reniform |
| Cleavage | {001} Perfect, {011} Distinct |
| Fracture | Subconchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 2.5 - 3 |
| Luster | Metallic |
| Streak | Black |
| Diaphaneity | Opaque |
| Specific gravity | 6.58-6.74 |
| References | [2][3][4][5] |
Penroseite is a rare selenide mineral 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/m3. Penroseite was first discovered in 1925 in a Bolivian rhyolite. It was named for Richard Penrose (1863–1931), an economic geologist.
Penroseite is a rare mineral found in the Pacajake mine in Bolivia. 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 group from the perspective of its structure, with a cubic space group (Bayliss, 1989).
Penroseite makes extensive solid solutions with other minerals. For example, penroseite can be a result of alteration process of many selenides, 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 and primary selenium-bearing sulfides, such as penroseite. Piretite forms as crusts in association with an orange masuyite-like U-Pb oxide on the surface of uraninite samples (Vochten, 1996).
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 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).