Sailaufite, (Ca, Na,)2 Mn3O2(AsO4) (CO3)3H2O, a new mineral from Hartkoppe hill, Ober-Sailauf (Spessart mountains, Germany), and its relationship to mitridatite-group minerals and pararobertsite (original) (raw)

Original paper

Wildner, Manfred; Tillmanns, Ekkehart; Andrut, Michael; Lorenz, Joachim

Abstract

Sailaufite, a new mineral with idealized composition CaNaMn3+3O2(AsO4)2(CO3)·3H2O, has been found at the locality Hartkoppe hill near Ober-Sailauf (Spessart mountains, Germany), associated with hausmannite, arseniosiderite, kutnahorite, dolomite, quartz, calcite, and Mn-calcite. It occurs as strongly intergrown, dark red-brown to black tabular crystals, often forming mammillated coatings on calcite and arseniosiderite. Mohs' hardness is ~3.5, the cleavage is perfect parallel (001), the tenacity is brittle with an uneven to conchoidal fracture. Crystals are optically biaxial negative, at 616 nm n = 1.757(5), = 0.004(1), 2V = 32(3)°, n (calc) = 1.806, n (calc) = 1.810. The strongest lines in the X-ray powder diffraction pattern are (dobs, I, hkl): 8.807, 100, 001; 5.654, 27, 130; 5.544, 17, 200; 2.936; 75, 003; 2.885, 19, 331; 2.816, 20, 332; 2.772, 36, 400; 2.514, 20, 133; 2.202, 55, 004. UV-VISIR spectroscopic measurements confirm the presence of AsO4 and CO3 groups as well as the absence of transition metal ions other than Mn3+ in sailaufite. The crystal structure of a small untwinned crystal fragment could be solved by direct methods from X-ray CCD data [monoclinic, space group Cm , a = 11.253(1), b = 19.628(1), c = 8.932(1) Å, = 100.05(1)°, Z = 6] and was refined on 8650 Fo2 to w R2 = 0.079 and R1 = 0.045. The structure contains characteristic nonamer rings of edge-sharing Mn3+O6 octahedra forming compact pseudotrigonal [Mn3+9O6(AsO4)6(CO3)3]9- sheets which are linked by layers composed of (Ca,Na, )O5(H2O)2 polyhedra and isolated water molecules. The octahedral nonamer rings correspond to those occuring in the Fe3+-phosphate mitridatite and bear strong similarities to the arrangement of Z-shaped octahedral chains in the Mn3+-phosphate pararobertsite.

Keywords

sailaufite • new mineral • crystal structure • arsenate • carbonate.