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Description, crystal structure, and paragenesis of krettnichite, PbMn³⁺₂(VO₄)₂(OH)₂, the Mn³⁺ analogue of mounanaite

¹ VIEPS, Department of Earth Sciences, VIC-3800 Monash University, Australia
² Laboratory of Chemical and Mineralogical Crystallography, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
³ Department of Mineralogy, The Natural History Museum, Cromwell Road, SW7 5BD London, UK
⁴ Institute of Geology, University of Copenhagen, Østervoldgade 10, 1350 Copenhagen, Denmark
⁵ Institute for Mineralogy and Petrography, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland
⁶ VIEPS School of Earth Sciences, The University of Melbourne, Parkville, VIC 3052, Australia

^* E-mail: joelb{at}mail.earth.monash.edu.au.

Krettnichite, PbMn³⁺₂(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 constituents), barian brackebuschite, cobaltoan pyrobelonite, and calcian mottramite (free crystals in vugs). Krettnichite is monoclinic, space group C2/m, Z = 2, a = 9.275(7) Å, b = 6.284(3) Å, c = 7.682(2) Å, ß = 117.97(4)°. Krettnichite is brown to black with red internal reflections. It has an excellent cleavage parallel to (001). A distinct cleavage intersecting (001) at high angle is visible in polished section. Polysynthetic twinning with composition plane (001) is common. Under plane polarised reflected light, krettnichite is slightly pleochroic, from very light grey 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 of this group with vanadate as the dominant anionic group, and is thus the Mn³⁺ analogue of mounanaite PbFe³⁺₂(VO₄)₂(OH)₂. The Fe content of krettnichite 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 corresponds to the extension of trans-trans chains of edge-sharing, Jahn-Teller distorted Mn³⁺O₆ octahedra in krettnichite as well as in brackebuschite. Pyrobelonite is always closely associated with barian brackebuschite. It is not known if true epitaxy occurs, but the chains of edge-sharing Mn³⁺O₆ octahedra of brackebuschite match edge-sharing chains of Mn²⁺O₆ octahedra in pyrobelonite.

Key-words: krettnichite, brackebuschite, mottramite, pyrobelonite, crystal structure, Jahn-Teller distortion, manganese deposit.

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