Description, crystal structure, and paragenesis of krettnichite, PbMn23+(VO4)(2)(OH)2, the Mn3+ analogue of mounanaite

Krettnichite PbMn23+(VO4)(2)(OH)(2), occurs as an accessory ore mineral and as free crystals in vugs within the hydrothermal manganite-quartz vein at the historic manganese deposit of Krettnich, Saarland, Germany. Other vein minerals include barite. ankerite, calcian mottramite (minor ore constituen ts), barian brackebuschite, cobaltoan pyrobelonite, and calcian mottramite (free crystals in vugs). Krettnichite is monoclinic, space group C2/m, Z = 2, a = 9.275(7) Angstrom. b = 6.284(3) Angstrom, c = 7.682(2) Angstrom, bet a = 117.97(4)degrees. Krettnichite is brown to black with red internal refl ections. It has an excellent cleavage parallel to (001). A distinct cleavag e intersecting (001) at high angle is visible in polished section. Polysynt hetic twinning with composition plane (001) is common. Under plane polarise d reflected light. krettnichite is slightly pleochroic. from very light gre y to light brownish grey, and the anisotropy is strong under crossed polars . Krettnichite is a member of the tsumcorite group. It is the second member o f this group with vanadate as the dominant anionic group, and is thus the M n3+ analogue of mounanaite PbFe23+(VO4)(2)(OH)(2). The Fe content of krettn ichite is generally low, and some crystals contain less than 0.001 atom Fe pfu. Epitactic intergrowths of krettnichite and brackebuschite are common, with parallel alignment of the b axes of both minerals. This direction corr esponds to the extension of trans-trans chains of edge-sharing, Jahn-Teller distorted Mn3+O6 octahedra in krettnichite as well as in brackebuschite, P yrobelonite is always closely associated with barian brackebuschite. It is not known if true epitaxy occurs, but the chains of edge-sharing Mn3+O6 oct ahedra of brackebuschite match edge-sharing chains of Mn2+O6 octahedra in p yrobelonite.