Skorpionite, Ca (sub 3) Zn (sub 2) (PO (sub 4) ) (sub 2) CO (sub 3) (OH) (sub 2) .H (sub 2) O, a new mineral from Namibia; description and crystal structure

Skorpionite, Ca (sub 3) Zn (sub 2) (PO (sub 4) ) (sub 2) CO (sub 3) (OH) (sub 2) .H (sub 2) O, is a new secondary mineral from the Skorpion zinc deposit in south-western Namibia. Associated minerals are tarbuttite, hydrozincite and gypsum. Skorpionite forms colorless 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/cm (super 3) ; D (calc.) = 3.17 g/cm (super 3) . Skorpionite is optically biaxial negative, alpha = 1.5884(10), beta = 1.6445 (calc.), gamma = 1.6455(10), 2V = 15.0 degrees (5) (589 nm); no dispersion. Optical orientation is Y and c = 26 degrees (in acute beta ); Z = b. Means of 17 sets of microprobe analyses gave CaO 30.89, ZnO 28.83, P (sub 2) O (sub 5) 25.49, CO (sub 2) (calc.) 7.96, H (sub 2) O (calc.) 6.52, total 99.69 wt.%. The empirical formula (based on 14 oxygen atoms pfu) is Ca (sub 3.05) Zn (sub 1.96) (PO (sub 4) ) (sub 1.99) (CO (sub 3) ) (sub 1.00) OH) (sub 2.06) .0.98H (sub 2) O. 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) Aa, beta = 92.73(2) degrees , V = 1156.9(4) Aa (super 3) , Z = 4. ZnO (sub 4) and PO (sub 4) tetrahedra are corner linked to corrugated sheets. The crystal structure forms a three-dimensional network of [Ca2 (sub 2) Zn (sub 2) (OH) (sub 2) (PO (sub 4) ) (sub 2) ] (super 0) and [Ca1(CO (sub 3) )(H (sub 2) O)] (super 0) layers that are linked by hydrogen bonds and by Ca-O bonds.