Abstract

Rappoldite, the Co-analogue of helmutwinklerite, and zincgartrellite, the Zn-dominant analogue of gartrellite, are two new members of the tsumcorite group. Both minerals are triclinic, their structures are closely related to the parent structure, i.e. the ‘tsumcorite type’ (C2/m, Z = 2). The lower symmetry is caused by two different crystal-chemical requirements. Order phenomena of the hydrogen bonds cause the ‘helmutwinklerite type’ (P1̅, Z = 4), ordering of Cu2+ and Fe3+ is responsible for the ‘gartrellite type’ (P1̅, Z = 1).

Rappoldite was found on samples from the Rappold mine near Schneeberg, Saxony, Germany. The new species forms red to red-brown prismatic and tabular crystals up to 1 mm long. Dcalc. = 5.28 g/cm3. 2Vz = 85(5)°, nx = 1.85 (calc.), ny = 1.87(2) and nz = 1.90(2); dispersion is distinct with r > v; orientation is Y ~|| [1̅20] and X ~ || c. The empirical formula derived from electron microprobe analyses is (Pb1.01Ca0.01)∑1.02(Co0.99Ni0.62Zn0.35Fe0.02)∑1.98[(AsO4)1.99(SO4)0.01]∑2.00[(OH)0.02(H2O)1.98]∑2.00 or Pb(Co,Ni)2(AsO4)2·2H2O. Single-crystal X-ray studies showed average C2/m symmetry. Weak superstructure reflections are responsible for triclinic symmetry and enlarged cell metrics (refined from powder data): a = 11.190(2) Å , b = 10.548(2) Å , c = 7.593(1) Å , α = 100.38(1)°, β = 109.59(2)°, γ = 98.96(1)°, V = 807.6 Å3, Z = 4. The superstructure results from the hydrogen-bond scheme, but faint streaks indicate some disorder. All investigated rappoldite crystals are twinned by reflection on (23̅0) which corresponds to the mirror plane of the average C2/m cell. Helmutwinklerite is isotypic with rappoldite and probably also with pure thometzekite; sulphatian thometzekite is monoclinic.

Zincgartrellite forms green-yellow rosette-like aggregates on samples from the Tsumeb mine, Namibia. The Dcalc. = 5.30 g/cm3. 2Vx = 87(5)°, nx = 1.91(2), ny = 1.94 (calc.) and nz = 1.97(2). Electron-microprobe analyses and Mössbauer data yielded the empirical formula (Pb0.97Ca0.04)∑1.01(Zn0.91Cu0.51Fe0.59Al0.03) ∑2.04[(AsO4)1.96(SO4)0.01] ∑1.97[(OH)0.81(H2O)1.31] ∑2.12 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. Helmutwinklerite has x near to 1 and is defined by a cell with fourfold volume. Single-crystal X-ray studies of zincgartrellite proved space group P1̅ caused by ordering of Fe3+ and Cu at one atomic site. Cell parameters (refined from powder data): a = 5.550(1) Å , b = 5.620(1) Å , c = 7.621(1) Å , α = 68.59(1), β = 69.17(1), γ = 69.51(1)°, V = 200.1 Å 3, Z = 1.

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