Köttigite
arsenate mineral
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 (1824–1892), a German chemist from Schneeberg, Saxony, who made the first chemical analysis of the mineral. It has the formula Zn3(AsO4)2·8H2O 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 Fe2+3(AsO4)2·8H2O; cobalt forms the distinctively coloured pinkish purple mineral erythrite Co3(AsO4)2·8H2O and nickel forms annabergite Ni3(AsO4)2·8H2O. Köttigite forms series with all three of these minerals and they are all members of the vivianite group.
The Vivianite Group is a group of monoclinic phosphates and arsenates with divalent cations. The group members are annabergite, arupite, babanekite, baricite, erythrite, hornesite, kottingite, manganhornesite, pakhomovskyite, parasymplesite and vivianite.
Structure
Köttigite belongs to the monoclinic crystal system, so it has two crystal axes, a and c, inclined to each other at angle β, and a third axis, the b axis, at right angles to both a and c. It belongs to point group 2/m, which means that it has a two-fold axis of rotational symmetry parallel to b, and a mirror plane perpendicular to it. The space group is C2/m, which means that the unit cell is centred on the C face.
Although zinc is the only transition metal that appears in the formula, köttigite usually contains significant quantities of cobalt and nickel, and these three elements are randomly distributed over the cation sites to form complex slabs perpendicular to the b axis.
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