Segregation and cation-ordering in sjögrenite and pyroaurite

Abstract

SummaryA specimen from Långban, Sweden, was found to consist largely of sjögrenite in which segregation and ordering of the metal cations had occurred, yielding an intergrowth of regions of two main types with compositions probably of or near Mg2/3Fe1/3(OH)2(CO3)1/6(H2O)0·4 and Mg12/13Fe1/13(OH)2(CO3)1/26(H2O)0·8. To a smaller extent segregation had proceeded further, giving regions probably approximating to the end-member Mg(OH)2.H2O. It is suggested that the names sjögrenite and pyroaurite be used for the 2H- and 3R- polytypes of this structure irrespective of Mg:Fe ratio, segregation, or cation ordering.