Sewardite

Sewardite is a rare arsenate mineral with formula of CaFe₂⁺³(AsO₄)₂(OH)₂.^[2] Sewardite was discovered in 1982 and named for the mineralogist, Terry M. Seward (born 1940), a professor of geochemistry in Zürich, Switzerland.^[2]

Properties

Sewardite is orthorhombic, which means in crystallographic terms, it contains 3 axes of unequal length, one "c" axis, and 1 "a" axis, and 1 "b" axis. The "c" axis runs vertically, and the "b" axis forms a 90 degree angle with the "c" axis; the "a" axis forms an angle that is less than 90 degrees. Its class structure in the crystal system, orthorhombic, is mmm (2/m 2/m 2/m) - dipyramidal. Sewardite can form platy to compact anhedral to subhedral masses up to 0.3 mm in size in a single specimen.^[3]

In terms of its optical properties, sewardite is weakly anisotropic, which means the velocity of light varies depending on the direction through the mineral. Its color in plane-polarized light is dark red, and it does not exhibit pleochroism, which means it does not appear to be a different color when observed at different angles under a polarizing petrographic microscope. Sewardite illustrates weak birefringence; birefringence is directly dependent on the material being anistropic, so since sewardite is weakly anistropic, it exhibits weak birefringence.^[4]

Occurrence

Sewardite has only been found at three locations, in the Tsumeb mine in Tsumeb, Namibia, Mina Ojuela, Mapimi, Durango, Mexico, and La Mur, Las Animas mine, Sonora, Mexico.^[2] At the site in Durango, Mexico, it occurs as a dark, reddish spherules and rosettes of very thin, flaky crystals.^[3]

This newly discovered mineral (confirmed as a species in 1998) has been determined as rare, since only 1–2 mg of it were found in the Tsumeb mine.^[3]

References