Benauite, a new mineral of the crandallite group from the Clara Mine in the Central Black Forest, Germany

Benauite is a new secondary mineral occurring at the Clara Mine in the Central Black Forest in association with fluorite, goethite and kidwellite. It forms radial aggregates up to 3 mm in diameter composed of poorly developed scaly crystals flattened on (0001). Colour yellow, brown, lustre vitreous to resinous, cleavage (0001) rather perfect, fracture conchoidal, hardness 31/2. D (meas.) 3.65 g/cm(3), D (calc.) 3.648 g/cm(3). Optical properties: n(epsilon) 1.862(5), n(omega) 1.872(5), nearly uniaxial negative or biaxial with 2 V up to 20 degrees, pleochroism: E nearly colourless, O yellow. Unit-cell dimensions: a 7.28(2), c 16.85(4) Angstrom, c:a = 2.315:1, space group probably R(3) over barm$. Strongest lines of the powder pattern: 5.91 (10) <10(1)over bar 1>, 3.65 (6) <10(2)over bar 0>, 3.06 (9) <11(2)over bar 3>, 2.25 (6) <10(1)over bar 7>. Chemical composition: CuO 0.03, ZnO 0.07, SrO 12.35, PbO 2.79, BaO 4.32, CaO 0.07, K2O 0.07, Al2O3 0.26, Fe2O3 40.85, P2O5 18.53, As2O5 0.78, SO3 6.79, H2O (by difference) 13.09, total 100.00% yielding the empirical formula (Sr0.067Ba0.16Pb0.07Ca0.01K0.01)(Sigma 0.92) (Fe2.90Al0.03)(Sigma 2.93)(PO4)(1.48) (AsO4)(0.04)(SO4)(0.48)(OH, H2O)(8.3) on the basis of P + As + S = 2, and simplified SrFe3H(PO4)(2)(OH)(6) or SrFe3(PO4)(2)(OH, H2O)(6). A mineral of similar composition, but richer in S is also described and considered to be a variety of benauite. The status of the members of the beudantite group is discussed in this connexion.