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

Abstract

Giftgrubeite, ideally CaMn2Ca2(AsO4)(2)(AsO3OH)(2).4H(2)O, 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, lollingite, 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 , 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 similar to 72 degrees and calculated 2V angle is 75.1 degrees; the refractive indices measured in white light are: alpha = 1.630(2), beta= 1.640(2) and gamma = 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 (Ca3.04Mn1.30Mg0.38Fe0.28)(Sigma 5.00)(AsO4)(1.99)(AsO3OH)(2).4H(2)O. Giftgrubeite is monoclinic, C2/c, Z = 4, with alpha = 18.495(7) , b = 9.475(4) , c = 9.986(4) , beta = 96.79(3)degrees and V = 1737.7(12) (3). The six strongest lines in the X-ray powder diffraction pattern are [d in (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 Ca3Mn1.30Mg0.70(AsO4)(2)(AsO3OH)(2)(H2O)(4). Giftgrubeite is an ordered intermediate member between villyaellenite, MnM(n)2Ca(2)(AsO3OH)(2)(AsO4)(2).4H(2)O and sainfeldite, CaCa2Ca2(AsO4)(2)(AsO3OH)(2).4H(2)O.