Penroseite

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).