brassite

{{Infobox mineral | name = Brassite | image = Rösslerite & Brassite-2.jpg | alt = | caption = Brassite (white) accompanied by rösslerite | category = Arsenate mineral | formula = Mg(AsO3OH)·4(H2O) | IMAsymbol=Bsi | strunz = 8.CE.15 | dana = 39.01.07.01 | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Pbca | unit cell = a = 7.472 Å, b = 10.891 Å, c = 16.585 Å; Z = 8 | color = White | colour = | habit = Cryptocrystalline crusts | twinning = | cleavage = {001} Perfect | fracture = Irregular/uneven, splintery | tenacity = Brittle | mohs = | luster = Silky, dull, earthy | streak = White | diaphaneity = Translucent | gravity = 2.28 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.531 nβ = 1.546 nγ = 1.562 | birefringence = 0.031 | pleochroism = Non-pleochroic | 2V = 80° | dispersion = r 2O | impurities = | alteration = | other = | prop1 = | prop1text = | references = }} Brassite is a rare arsenate mineral with the chemical formula Mg(AsO3OH)·4(H2O). It was named brassite, in 1973, to honor French chemist Réjane Brasse, who first synthesized the compound. The type locality for brassite is Jáchymov of the Czech Republic.

It occurs as an alteration of magnesium carbonate minerals by arsenic bearing solutions. It occurs associated with pharmacolite, picropharmacolite, weilite, haidingerite, rauenthalite, native arsenic, realgar and dolomite.