Vanackerite, a new lead cadmium arsenate of the apatite supergroup from Tsumeb, Namibia
Schlüter, Jochen; Malcherek, Thomas; Gebhard, Georg
published: Jan 1, 2016
ArtNo. ESP154019301005, Price: 29.00 €
The mineral vanackerite (IMA 2011-114), Pb4 Cd(AsO4)3 Cl is a new member of the apatite supergroup of minerals and a Cd-substituted derivative of mimetite. Vanackerite is trigonal with space group P¯3 and its crystal structure is obtained from the hexagonal mimetite structure by site-splitting of one of the Pb-positions, accompanied by rotation of the arsenate tetrahedra. Single crystal X-ray diffraction yields a = 10.0279(3) Å, c = 7.2965(2) Å, V = 635.43(4) Å3 and Z = 2. The strongest X-ray powder reflections are as follows, dobs. (Å) (I;hkl): 2.982 (100; 211,112, 300), 3.290 (34; 102, 3 ¯20), 2.067 (16; 222) and 1.944 (11; 3¯13). Chemical analyses gave (wt.%): PbO 64.44, CdO 8.82, As2 O5 23.59, Cl 1.51, O = Cl – 0.34, sum 98.02 wt.%. The resulting empirical formula based on 8 cations pfu is Pb4.10 Cd0.98 As2.92 O12.07 Cl0.61. The new mineral was collected in 1980 and since then mistaken for mimetite. The sample originates from the second oxidation zone of the Tsumeb ore deposit, Tsumeb, Namibia. Vanackerite occurs as light yellow rosettes of thin pseudohexagonal tabular crystals up to 5 mm in size on bluish green thometzekite, further associated minerals are anglesite and gypsum. Its calculated density is 7.28, the calculated mean refractive index is 2.04.