| Sewardite | |
|---|---|
| General | |
| Category | Arsenate minerals |
| Formula | CaFe2+3(AsO4)2(OH)2 |
| IMA symbol | Sew1 |
| 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 |
| Identification | |
| 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 | 23 |
Sewardite is a rare arsenate mineral with formula of CaFe3+2(AsO4)2(OH)2.3 Sewardite was discovered in 1982 and named for the mineralogist, Terry M. Seward (born 1940), a professor of geochemistry in Zürich, Switzerland.3
Properties
Sewardite is orthorhombic, which means that it contains three axes of unequal length, a, b, and c, which are all at 90° to one another. Its class structure 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.4
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 because it is weakly anisotropic.5
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.3 At the site in Durango, Mexico, it occurs as a dark, reddish spherules and rosettes of very thin, flaky crystals.4
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.4
References
References
- Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- "Sewardite Mineral Data." http://webmineral.com/data/Sewardite.shtml. Accessed 30 September 2010.
- Mindat.org
- 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
- "Mineral General Info." http://www.mineralatlas.com/mineral%20optical%20descriptions/S/sewarditeopt.htm. Accessed 7 November 2010.