Abstract

Ga-rich plumbogummite, (Pb0.87,Ca0.13)Σ1.00H(Al1.95,Ga1.05)Σ3.00(PO4)2(OH)6, from Tsumeb, Namibia, has rhombohedral symmetry, space group R

\({\bar{3}}\)
m, with the cell parameters a = 7.0752(19) Å, c = 16.818(4) Å and V = 729.1(3) Å3. The crystal structure has been refined to R1 = 2.05%. Ga-rich plumbogummite has an alunite-type structure comprised of a rhombohedral stacking of (001) composite layers of corner-shared (Al,Ga)O6 octahedra and PO4 tetrahedra, with Pb atoms occupying icosahedrally coordinated sites between the layers. The Pb and H positions are discussed. Ga-rich plumbogummite is nonpleochroic, uniaxial (+), with indices of refraction, ϵ = 1.742(3) and ω = 1.722(3), determined in white light. The five strongest powder-diffraction lines [d in Å, (I/Io), (hkl)] are: 2.995, (100), (113); 5.766, (95), (101); 2.236, (43), (107, 122); 3.539, (38), (110); 1.919 (32), (303, 033).

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