Holdawayite, a new manganese hydroxyl-carbonate from the Kombat Mine, Namibia

Holdawayite, a new mineral occurring at the Kombat mine, Namibia, ideal formula, Mn6(CO3)2(OH)7(Cl,OH) is monoclinic, space group C2/m, with a 23.437(5), b 3.3137(3), c 16.618(6) Angstrom, beta 111.15(2) degrees , Z = 4. The most intense powder XRD lines are 10.93(100),5.459(80), 3.879(70), 1.789(70), 2.690(60) Angstrom. It occurs principally as subhedral grains in Mn ores and rarely as fibrous, silky aggregates. It is pink and translucent and has a vitreous lustre. The observed and calculated densities are 3.19(4) and 3.24(3) g/cm3, respectively; optical properties include 2Valpha 12(3) degrees (meas.) 18 degrees (calc.); alpha 1.644(1), beta 1.719(1), gamma 1.721(1). Holdawayite forms coarse veins cutting silicate-facies Mn-bearing rocks with assemblages including humite- and/or leucophoenicite-group minerals. Idealized Cl-free holdawayite, Mn3(CO3)(OH)4, may form by hydration of rhodochrosite and in turn may oxidize to higher oxides and hydroxides of Mn. It has been named in honour of Dr. Michael J. Holdaway, Professor of petrology at Southern Methodist University.