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Category Arsenate minerals
(repeating unit)
Strunz classification 8.BH.30
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Cccm
Unit cell a = 16.461 Å, b = 7.434 Å,
c = 12.131 Å; Z = 8
Formula mass 464.68 g/mol
Color Dark red, lighter red orange
Crystal habit Platy aggregates, anhedral grains
Cleavage {100} and {011} imperfect
Fracture Splintery - thin elongated fractures
Mohs scale hardness 3.5
Luster Vitreous (glassy)
Streak Reddish brown
Diaphaneity Translucent
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.[2] Sewardite was discovered in 1982 and named for the mineralogist, Terry M. Seward (born 1940), a professor of geochemistry in Zürich, Switzerland.[2]


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]


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]