köttigite

{{Infobox mineral | name = Köttigite | category = Arsenate mineral | boxwidth = 24 | boxbgcolor = | image = Kottigite-249705.jpg | imagesize = 260px | caption = | formula = | IMAsymbol = Köt | molweight = 618.13 g/mol | strunz = 8.CE.40 (10 ed.) VII/C.13-90 (8 ed.) | dana = 40.3.6.5 | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = C2/m | colour = Colourless, pink, red, red-orange or brown; light rose-pink in transmitted light | habit = Crystals prismatic [001] and flattened {010}. Also in crusts with a crystalline surface and fibrous structure | twinning = | cleavage = Perfect on {010} | fracture = Fibrous fracture yields a silky lustre | tenacity = Flexible | mohs = to 3 | lustre = Resinous or waxy, silky on fracture | refractive = nα = 1.622nβ = 1.638nγ = 1.671 | opticalprop = Biaxial (+) | birefringence = δ = 0.049 | pleochroism = Visible. X, Y = colourless, Z = pale red. | streak = Reddish-white to white | gravity = 3.33 | melt = | fusibility = | diagnostic = | solubility = Soluble in acids | diaphaneity = Translucent | other = Not fluorescent | references = }}

Köttigite is a rare hydrated zinc arsenate which was discovered in 1849 and named by James Dwight Dana in 1850 in honour of Otto Friedrich Köttig (18241892), a German chemist from Schneeberg, Saxony, who made the first chemical analysis of the mineral. It has the formula and it is a dimorph of metaköttigite, which means that the two minerals have the same formula, but a different structure: köttigite is monoclinic and metaköttigite is triclinic. There are several minerals with similar formulae but with other cations in place of the zinc. Iron forms parasymplesite ; cobalt forms the distinctively coloured pinkish purple mineral erythrite and nickel forms annabergite . Köttigite forms series with all three of these minerals and they are all members of the vivianite group.