Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Nicolas Meisser, Jakub Plášil, Thierry Brunsperger, Cédric Lheur, Radek Škoda

Giftgrubeite, CaMn₂Ca₂(AsO₄)₂(AsO₃OH)₂·4H₂O, a new member of the hureaulite group from Sainte-Marie-aux-Mines, Haut-Rhin Department, Vosges, France

Journal of Geosciences, volume 64 (2019), issue 1, 73 - 80

DOI: http://doi.org/10.3190/jgeosci.276

Giftgrubeite, ideally CaMn₂Ca₂(AsO₄)₂(AsO₃OH)₂·4H₂O, is a new mineral occurring at the Giftgrube Mine, St Jacques vein, Rauenthal, Sainte-Marie-aux-Mines, Haut-Rhin Department, Grand Est, France and named after the type-locality. Giftgrubeite is mostly associated with Mn-bearing calcite, native arsenic, löllingite, and picropharmacolite. It is a recent secondary mineral, formed by alteration of the arsenical vein minerals after mining. Giftgrubeite occurs in colorless, rarely pearl white to pale yellow rosettes of brittle tabular crystals flattened on {1 0 0} and up to 0.2 mm in size. Hardness (Mohs) is 3 ½, D_meas is 3.23(2) g·cm^-3, D_calc is 3.24 g·cm^-3. The new mineral is biaxial negative without pleochroism. Measured 2V angle is ~72° and calculated 2V angle is 75.1°; the refractive indices measured in white light are: α = 1.630(2), β= 1.640(2) and γ = 1.646(2). The most prominent Raman bands are at 902, 885, 864, 851, 824, 797 and 759 cm^-1. The empirical chemical formula is (Ca_3.04Mn_1.30Mg_0.38Fe_0.28)_Σ5.00(AsO₄)_1.99(AsO₃OH)₂·4H₂O. Giftgrubeite is monoclinic, C2/c, Z = 4, with a = 18.495(7) Å, b = 9.475(4) Å, c = 9.986(4) Å, β = 96.79(3)° and V = 1737.7(12) ų. The six strongest lines in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 3.33 (100)(-2 2 2), 3.18 (80)(2 2 2), 2.414 (60)(7 1 1), 4.80 (50)(-3 1 1), 4.65 (50)(-2 0 2) and 3.05 (50)(1 1 3). The structure of giftgrubeite was solved from the crystal retrieved from the type specimen by the charge-flipping algorithm. Giftgrubeite contains a well-known structure type parent to the hureaulite group of minerals, which is based upon an octahedral edge-sharing pentamers of M²⁺-polyhedra; pentamers linked into a loose framework by sharing corners with octahedra in adjacent pentamers and further by AsO₄ and AsO₃OH tetrahedra. There are three distinct octahedral sites: M1, M2, and M3. In the case of giftgrubeite, two of the M sites were found to be fully occupied by Ca; namely M1 and M3. The M2 site was then found to contain Mg besides dominant Mn. Considering the refined site occupancies, the structural formula for giftgrubeite is Ca₃Mn_1.30Mg_0.70(AsO₄)₂(AsO₃OH)₂(H₂O)₄. Giftgrubeite is an ordered intermediate member between villyaellenite, MnMn₂Ca₂(AsO₃OH)₂(AsO₄)₂·4H₂O and sainfeldite, CaCa₂Ca₂(AsO₄)₂(AsO₃OH)₂·4H₂O.