Arsenites related to layer silicates; manganarsite, the arsenite analogue of manganpyrosmalite, and unnamed analogues of friedelite and schallerite from Laangban, Sweden

Manganarsite, ideally Mn₃As₂O₄(OH)₄, is a new mineral from Långban, Sweden. X-ray diffraction data imply it has space group P31m, P31m or P312, with a = 11.451(6), c = 7.252(6) Å, Z = 4, although optical data imply a lower symmetry. The powder-difFraction pattern has principal reflections (d, I/I₀, hkl) 3.62,50,002; 2.662,100,221; 1.845,70,223,331; 1.652,60,600; 1.531,40,224, 503. There is a perfect {001} cleavage; hardness = 3; D_calc = 3.60, D_obs = 3.64 g/cm³.

Two other polymorphs are described, which together with manganarsite are analogous to the three principal members of the friedelite group, manganpyrosmalite, schallerite, and friedelite. Manganarsite and its polymorphs are 7-Å layered structures that apparently differ in the stacking sequence of pyrochroite layers, as is true for members of the friedelite family. However, indirect evidence implies that manganarsite and its polymorphs have a layer based on arsenite chains or rings in place of the tetrahedral sheet of friedelite.