| Sewardite | |
|---|---|
| General | |
| Category | Arsenate minerals |
| Chemical formula | CaFe2+3(AsO4)2(OH)2 |
| Strunz classification | 08.BH.30 |
| Crystal symmetry | Orthorhombic (2/m 2/m 2/m) - dipyramidal |
| Unit cell | a = 16.461 Å, b = 7.434 Å, c = 12.131 Å; Z = 8 |
| Identification | |
| Molar mass | 464.68 g |
| Color | Dark red, lighter red orange |
| Crystal habit | Platy, aggregates, anhedral Grains |
| Crystal system | Orthorhombic space group = C ccm |
| Cleavage | {100} and {011} imperfect |
| Fracture | Splintery - thin elongated fractures |
| Mohs scale hardness | 3.5 |
| Luster | Vitreous (glassy) |
| Streak | Reddish brown |
| Diaphaneity | Transluscent |
| Specific gravity | 4.16 |
| Optical properties | Weak Anistropic |
| Refractive index | 1.94 calculated |
| Birefringence | Weak |
| Pleochroism | None |
| References | [1][2] |
Sewardite is a rare arsenate mineral with formula of CaFe2+3(AsO4)2(OH)2.[3] Sewardite was discovered in 1982 and named for the mineralogist, Terry M. Seward (1940-), a professor of geochemistry in Zürich, Switzerland.[3]
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. It's 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.[4]
In terms of its optical properties, sewardite is weak 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.[5]
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] The Tsumeb Mine is the most prolific mine in the world, and occurs in a geologic setting of the subduction belt of Namibia, which produces the conditions in which sewardite occurs. At the site in Durango, Mexico, it occurs as a dark, reddish spherules and rosettes of very thin, flaky crystals that would be compacted together in sparkling masses.
This newly discovered mineral (confirmed as a species in 1998) has been determined as rare, since only 1-2 mg of it have been found on Earth. Since it's such a rarity, any use found for the mineral would be very impractical due to its existing quantity.[6]