{{Short description|Rare arsenate mineral}} {{Infobox mineral | name = Sewardite | category = Arsenate minerals | boxwidth = | boxbgcolor = | image = | imagesize = | caption = | formula = CaFe<sub>2</sub><sup>+3</sup>(AsO<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub> | IMAsymbol = Sew<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = 464.68 g/mol | strunz = 8.BH.30 | system = Orthorhombic | class = Dipyramidal (mmm) <br/>H-M symbol: (2/m 2/m 2/m) | symmetry = ''Cccm'' | unit cell = a = 16.461&nbsp;Å, b = 7.434&nbsp;Å, <br/>c = 12.131&nbsp;Å; Z&nbsp;=&nbsp;8 | color = Dark red, lighter red orange | habit = Platy aggregates, anhedral grains | twinning = | cleavage = {100} and {011} imperfect | fracture = Splintery – thin elongated fractures | tenacity = | mohs = 3.5 | luster = Vitreous (glassy) | refractive = 1.94 calculated | opticalprop = Weak Anistropic | birefringence = Weak | pleochroism = None | streak = Reddish brown | gravity = 4.16 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Translucent | other = | references = <ref>"Sewardite Mineral Data." http://webmineral.com/data/Sewardite.shtml. Accessed 30 September 2010.</ref><ref name=Mindat>[http://www.mindat.org/min-11128.html Mindat.org]</ref> }} '''Sewardite''' is a rare arsenate mineral with formula of CaFe<sup>3+</sup><sub>2</sub>(AsO<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>.<ref name=Mindat/> Sewardite was discovered in 1982 and named for the mineralogist, Terry M. Seward (born 1940), a professor of geochemistry in Zürich, Switzerland.<ref name=Mindat/>

== 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&nbsp;mm in size.<ref name=Canmin>[http://rruff.geo.arizona.edu/doclib/cm/vol40/CM40_1191.pdf 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]</ref>

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.<ref>"Mineral General Info." http://www.mineralatlas.com/mineral%20optical%20descriptions/S/sewarditeopt.htm. Accessed 7 November 2010.</ref>

==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.<ref name=Mindat/> At the site in Durango, Mexico, it occurs as a dark, reddish spherules and rosettes of very thin, flaky crystals.<ref name=Canmin/>

This newly discovered mineral (confirmed as a species in 1998) has been determined as rare, since only 1–2&nbsp;mg of it were found in the Tsumeb mine.<ref name=Canmin/>

==References== {{Reflist}}

Category:Arsenate minerals Category:Calcium minerals Category:Iron(III) minerals Category:Orthorhombic minerals Category:Minerals in space group 66