sewardite

{{Infobox mineral | name = Sewardite | category = Arsenate minerals | boxwidth = | boxbgcolor = | image = | imagesize = | caption = | formula = CaFe2+3(AsO4)2(OH)2 | IMAsymbol = Sew | molweight = 464.68 g/mol | strunz = 8.BH.30 | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Cccm | unit cell = a = 16.461 Å, b = 7.434 Å, c = 12.131 Å; Z = 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 = }} Sewardite is a rare arsenate mineral with formula of CaFe3+2(AsO4)2(OH)2. Sewardite was discovered in 1982 and named for the mineralogist, Terry M. Seward (born 1940), a professor of geochemistry in Zürich, Switzerland.

== 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.