The crystal chemistry of the sartorite group minerals from Lengenbach, Binntal, Switzerland - a HRTEM study

Abstract

The crystal chemistry of the sartorite group of minerals is reviewed and structures are treated in terms of compositional modulations associated with Pb and As ordering. The minerals have modular structures based on the intergrowth N = 4 and N = 3 units, with N = 4 units dominated the more Pb-rich minerals. High-resolution transmission electron microscopy was used to investigate the nature of long range order-disorder of structural modules. The more Pb-rich members of the group: dufrénoysite (N = 4), rathite (N = 4), and liveingite (N = 3.75) were found to be perfectly ordered with no evidence of polytypism or polysomatic disorder. The compositional region between liveingite and baumhauerite is dominated by the occurrence of long period homologues with 10 and 12 units in the structural repeat. Disordered intergrowths of N = 4 and N = 3 units occur in this compositional region. Baumhauerite (N = 3.5) exhibits extensive polytypism, both ordered and disordered and this is thought to be associated with periodic inversions of the Pb/As order about c. Sartorite (N = 3) exhibits a complex modulated structure that is associated with the inherent mismatch of As and Pb polyhedra within the structural modules. Critical evaluation of the structural data on rathite suggests that it is part of a complex solid solution between dufrénoysite and a hypothetical end-member Pb8(Ag,Tl)8As24S40, The existence of rathite III and its relationship to rathite are discussed. The paragenetic relationships between the sartorite minerals at the Lengenbach Quarry, Binntal, Switzerland are also discussed.