On the symmetry of tsumcorite group minerals based on the new species rappoldite and zincgartrellite

Rappoldite, the Go-analogue of helmutwinklerite, and zincgartrellite, the Z n-dominant analogue of gartrellite, are two new members of the tsumcorite g roup. Both minerals are triclinic, their structures are closely related to the parent structure, i.e. the 'tsumcorite type' (C2/m, Z = 2). The lower s ymmetry is caused by two different crystal-chemical requirements. Order phe nomena of the hydrogen bonds cause the 'helmutwinklerite type' (PI, Z = 4), ordering of Cu2+ and Fe3+ is responsible for the 'gartrellite type' (P (1) over bar, Z = I). Rappoldite was found on samples from the Rappold mine near Schneeberg, Saxo ny, Germany. The new species forms red to red-brown prismatic and tabular c rystals up to 1 mm long. D-calc. = 5.28 g/cm(3). 2V(z) = 85(5)degrees, n(x) = 1.85 (calc.), n(y) = 1.87(2) and n(z) = 1.90(2); dispersion is distinct with r > v; orientation is Y similar to parallel to [(1) over bar 20] and X similar to parallel to c. The empirical formula derived from electron micr oprobe analyses is (Pb1.01Ca0.01)(Sigma1.02)(Co0.99Ni0.62Zn0.35Fe0.02)(Sigm a1.98)[(AsO4)(1.99)(SO4)(0.01)](Sigma2.00)[(OH)(0.02)(H2O)(1.98)](Sigma2.00 ) or Pb(Co,Ni)(2)(AsO4)(2). 2H(2)O. Single-crystal X-ray studies showed ave rage C2/m symmetry. Weak superstructure reflections are responsible for tri clinic symmetry and enlarged cell metrics (refined from powder data): a = 1 1.190(2) Angstrom, b = 10.548(2) Angstrom, c = 7.593(1) Angstrom, alpha = 1 00.38(1)degrees beta = 109.59(2)degrees gamma = 98.96(1)degrees, V = 807.6 Angstrom (3), Z = 4. The superstructure results from the hydrogen-bond sche me, but faint streaks indicate some disorder. All investigated rappoldite c rystals are twinned by reflection on (2 (3) over bar0) which corresponds to the mirror plane of the average C2/m cell. Helmutwinklerite is isotypic wi th rappoldite and probably also with pure thometzekite; sulphatian thometze kite is monoclinic. Zincgartrellite forms green-yellow rosette-like aggregates on samples from the Tsumeb mine, Namibia. The D-calc. = 5.30 g/cm(3). 2V(x) = 87(5)degrees, n(x) = 1.91(2), n(y) = 1.94 (calc.) and n(z) = 1.97(2). Electron-microprob e analyses and Mossbauer data yielded the empirical formula (Pb0.97Ca0.04)( Sigma1.01)(Zn0.91Cu0.51Fe0.59Al0.03)(Sigma2.04)[(AsO4)(1.96)(SO4)(0.01)](Si gma1.97)[(OH)(0.81)(H2O)(1.31)](Sigma2.12) or Pb(Zn,Fe,Cu)(2)(AsO4)(2)(H2O, OH)(2). The structural formula is Pb(ZnxFe1-x)(ZnxCu1-x(AsO4)(2)(OH)(1-x) ( H2O)(1+x) With 0.4 < x < 0.8. Gartrellite is defined by x < 0.4. Helmutwink lerite has x near to 1 and is defined by a cell with fourfold volume. Singl e-crystal X-ray studies of zincgartrellite proved space group P<(1)over bar > caused by ordering of Fe3+ and Cu at one atomic site. Cell parameters (re fined from powder data): a = 5.550(1) Angstrom, b = 5.620(1) Angstrom, c = 7.621(1) Angstrom, alpha = 68.59(1), beta = 69.17(1), gamma = 69.51(1)degre es, V = 200.1 Angstrom (3), Z = 1.