The crystal structure of Ga-rich plumbogummite from Tsumeb, Namibia

Abstract

ABSTR ACTGa-rich plumbogummite, (Pb 0.87 ,Ca 0.13 ) S1.00 H(Al 1.95 ,Ga 1.05 ) S3.00 (PO 4 ) 2 (OH) 6 , from Tsumeb, Namibia, has rhombohedral symmetry, space group R3m, with the cell parameters a = 7.0752(19) Å , c = 16.818(4) Å and V = 729.1(3) Å 3 . The crystal structure has been refined to R 1 = 2.05%. Ga-rich plumbogummite has an alunite-type structure comprised of a rhombohedral stacking of (001) composite layers of corner-shared (Al,Ga)O 6 octahedra and PO 4 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, e = 1.742 (3) and o = 1.722(3), determined in white light. The five strongest powder-diffraction lines [d in Å , (I/I o ), (hkl)] are: 2.995

Hydrocerussite-related minerals and materials: structural principles, chemical variations and infrared spectroscopy

“…One may hypothesize that cation disorder in the interleaved sheets occurs preferentially when Pb 2+ cations with diffuse electron distribution are present (as in hydrocerussite, grootfonteinite, somersetite), but ordered cations are present when alkali cations are present, such as Na + or K + (abellaite-type phases and slag phase from Lavrion). A similar phenomenon is observed in the alunite supergroup minerals; for example the Pb has to move off the origin in the structure of plumbogummite to accommodate the lone pair (Mills et al, 2009), but K remains intact in the structure of jarosite (Mills et al, 2013). Another example is a pair of lead-tellurium oxysalts: eckhardite (Kampf et al, 2013) and andychristyite (Kampf et al, 2016).…”

Lead-tellurium oxysalts from Otto Mountain near Baker, California: V. Timroseite, Pb2Cu52+(Te6+O6)2(OH)2, and paratimroseite, Pb2Cu42+(Te6+O6)2(H2O)2, two new tellurates with Te-Cu polyhedral sheets

“…7). All of the Pb coordinations in timroseite and paratimroseite are lopsided as is typical for Pb 2+ with stereoactive 6s 2 lone electron pairs (e.g., Moore 1988;Cooper and Hawthorne 1994;Kharisun et al 1997;Mills et al 2009a). In fact, the Pb atoms in the structures of all seven recently discovered new minerals from Otto Mountain exhibit this feature.…”

Sb5+ and Sb3+ substitution in segnitite: A new sink for As and Sb in the environment and implications for acid mine drainage

“…It may be that the small amount of S [the general average S-O in sulfates is 1.459; Hawthorne et al (2000)] counteracts the effect of the Sb 3+ . It is also noteworthy that the BVS for Pb and O2 show overbonding, which is typical in the Pb-rich members of the alunite supergroup (Mills et al 2009d). …”