Original paper

Skorpionite, Ca3Zn2(PO4)2CO3(OH)2H2O, a new mineral from Namibia: description and crystal structure

Krause, Werner; Effenberger, Herta; Bernhardt, Heinz-Jürgen; Medenbach, Olaf

Abstract

Skorpionite, Ca3Zn2(PO4)2CO3(OH)2·H2O, is a new secondary mineral from the Skorpion zinc deposit in south-western Namibia. Associated minerals are tarbuttite, hydrozincite and gypsum. Skorpionite forms colourless needle-like crystals of monoclinic symmetry that are elongated parallel to [001] with dominant {100} and {110}; additional forms are {111}, {221}, {131} and {−101}. D (meas.) = 3.15(3) g/cm3; D (calc.) = 3.17 g/cm3. Skorpionite is optically biaxial negative, α = 1.5884(10), β = 1.6445 (calc.), γ = 1.6455(10), 2V = 15.0°(5)(589 nm); no dispersion. Optical orientation is Y∧c = 26° (in acute β); Z = b. Means of 17 sets of microprobe analyses gave CaO 30.89, ZnO 28.83, P2O5 25.49, CO2 (calc.) 7.96, H2O (calc.) 6.52, total 99.69 wt.%. The empirical formula (based on 14 oxygen atoms pfu) is Ca3.05Zn1.96(PO4)1.99(CO3)1.00(OH)2.06·0.98H2O. Single-crystal X-ray investigations gave space group C2/c, with unit-cell parameters a = 19.045(3), b = 9.320(2), c = 6.525(1) Å, β = 92.73(2)°, V = 1156.9(4) Å3, Z = 4. ZnO4 and PO4 tetrahedra are corner linked to corrugated sheets. The crystal structure forms a three-dimensional network of [Ca22Zn2(OH)2(PO4)2]0 and [Ca1(CO3)(H2O)]0 layers that are linked by hydrogen bonds and by Ca-O bonds.

Keywords

skorpionitenew mineralskorpion minenamibiamicroprobe analysiscrystal structurecalcium zinc carbonate-phosphate hydroxyhydrate