Joachim Lorenz

Mineralogy of complex Co-Ni-Bi vein mineralization, Bieber deposit, Spessart, Germany

Abstract Post-Variscan vein-type Co-Ni-Bi ores of the Bieber deposit, Spessart mountains, Germany, which are related to the Permian Kupferschiefer, have been investigated by ore microscopy, X-ray powder diffraction and electron-probe microanalysis. The samples contain a variety of ore minerals, notably skutterudite, native bismuth, cobaltite, alloclasite, niccolite, maucherite, gersdorffite, rammelsbergite/ pararammelsbergite, safflorite, loellingite and emplectite. The ores display structures indicative of multiple brecciation and complex zoned arsenide assemblages. Three sequential stages of deposition are identified, which are (1) the Cu stage, (2) the main Co-Ni-Bi stage, and (3) the late stage. The arsenide minerals, notably skutterudite, diarsenides and sulpharsenides, show a large range of compositional variation in Co-Ni-Fe space. A relatively limited number of skutterudite and diarsenide compositions lie outside the compositional fields established in the literature. Skutterudite and diarsenides are characterized by a significant substitution of As by S up to 0.44 a.p.f.u. and 0.31 a.p.f.u., respectively, which is larger than the range previously reported for these minerals. Sulpharsenide compositions can be grouped into three populations, which conform to cobaltian arsenopyrite, cobaltite and gersdorffite. They display highly variable As/S ratios between 0.95:1.00 and 1.29:0.73, consistent with experimental data. Estimates of the formation temperatures, based on the presence of dendritic native bismuth and emplectite, are in the range 100-300°C, similar to different post-Variscan mineralization styles widespread in Central Europe. Comparison of the Co-Ni-Bi vein assemblage with the framework of available paragenetic information and radiometric age data for regional mineralization events indicates an age of mineralization of ~150-160 Ma for the Bieber deposit.

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

Sailaufite, a new mineral with idealized composition CaNaMn 3+ 3 O 2 (AsO 4 ) 2 (CO 3 ).3H 2 O, 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. Mohs9 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 α = 1.757(5), δ β,γ = 0.004(1), 2V = 32(3)°, nβ(calc) = 1.806, nγ(calc) = 1.810. The strongest lines in the X-ray powder diffraction pattern are (d obs , 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-VIS-IR spectroscopic measurements confirm the presence of AsO 4 and CO 3 groups as well as the absence of transition metal ions other than Mn 3+ 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) A, β = 100.05(1)°, Z = 6] and was refined on 8650 F 2 o to w R2 = 0.079 and R1 = 0.045. The structure contains characteristic nonamer rings of edge-sharing Mn 3+ O 6 octahedra forming compact pseudotrigonal [Mn 3+ 9 O 6 (AsO 4 ) 6 (CO 3 ) 3 ] 9− sheets which are linked by layers composed of (Ca,Na,□)O 5 (H 2 O) 2 polyhedra and isolated water molecules. The octahedral nonamer rings correspond to those occuring in the Fe 3+ -phosphate mitridatite and bear strong similarities to the arrangement of Z-shaped octahedral chains in the Mn 3+ -phosphate pararobertsite.

Okruschite, Ca2Mn2+5Be4(AsO4)6(OH)4 · 6H2O, a new roscherite-group mineral from Sailauf, Bavaria, Germany

The new mineral okruschite (IMA No. 2013–047), the arsenate analogue of roscherite, was discovered in a hydrothermal vein cross-cutting rhyolite exposed in the Fuchs quarry, near Sailauf, Spessart Mountains, Bavaria, Germany. Associated minerals are braunite, Mn-bearing calcite and arseniosiderite. Okruschite forms tabular aggregates up to 0.15 × 0.3 × 0.3 mm in size of curved and somewhat misaligned laths. It is white, semitransparent; lustre is vitreous. D meas = 3.33(2), D calc = 3.340 g/cm 3 . Okruschite is optically biaxial (−), α = 1.671(3), β = 1.682(2), γ = 1.687(3), 2 V meas = 65(5)°. The infrared spectrum is given. The chemical composition is (electron microprobe, B and Li by ICP MS, H 2 O by gas chromatography of ignition products, wt. %): Li 2 O 0.04, BeO 7.70, MgO 1.68, CaO 8.28, MnO 16.27, FeO 4.89, Al 2 O 3 0.22, As 2 O 5 51.11, H 2 O 11.0, total 101.19. The empirical formula based on 34 O atoms is: Ca 1.99 (Mn 3.09 Fe 0.92 Mg 0.56 Al 0.06 Li 0.04 ) ∑4.67 Be 4.15 (AsO 4 ) 5.99 (OH) 3.64 · 6.40H 2 O. The simplified formula is Ca 2 Mn 2+ 5 Be 4 (AsO 4 ) 6 (OH) 4 · 6H 2 O. Okruschite is monoclinic, C 2/ c , a = 16.33(4), b = 12.03(3), c = 6.93(1) A, β = 94.84(5)°, V = 1357(4) A 3 , Z = 2. The strongest lines of the powder X-ray diffraction pattern [ d , A ( I , %) ( hkl )] are: 9.68 (39) (110), 4.95 (34) (310), 4.17 (34) (−311), 3.25 (100) (−202, 330), 3.11 (32) (−421), 2.841 (27) (240), 2.711 (26) (600), 1.726 (26) (461, −552, 004). Okruschite is named after Professor Martin Okrusch, from Germany. Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.