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]

Sewardite
General
CategoryArsenate minerals
Formula
(repeating unit)		CaFe2+3(AsO4)2(OH)2
Strunz classification8.BH.30
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupCccm
Unit cella = 16.461 Å, b = 7.434 Å,
c = 12.131 Å; Z = 8
Identification
Formula mass464.68 g/mol
ColorDark red, lighter red orange
Crystal habitPlaty aggregates, anhedral grains
Cleavage{100} and {011} imperfect
FractureSplintery - thin elongated fractures
Mohs scale hardness3.5
LusterVitreous (glassy)
StreakReddish brown
DiaphaneityTranslucent
Specific gravity4.16
Optical propertiesWeak Anistropic
Refractive index1.94 calculated
BirefringenceWeak
PleochroismNone
References[1][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]

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References
  1. "Sewardite Mineral Data." http://webmineral.com/data/Sewardite.shtml. Accessed 30 September 2010.
  2. Mindat.org
  3. Roberts, A.C., Cooper, M.A., Hawthorne, F.C., Criddle, A.J., and Striddle, J.A.R. (2002) Sewardite, CaFe32+(AsO4)2(OH)2, the Ca-analog of carminite, from Tsumeb, Namibia: description and crystal structure. Canadian Mineralogist, 40, 1191-1198
  4. "Mineral General Info." http://www.mineralatlas.com/mineral%20optical%20descriptions/S/sewarditeopt.htm. Accessed 7 November 2010.
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