Crystal chemistry of julgoldite, a mineral series of the pumpellyite group: re-investigation of Fe distribution and hydrogen-bonding

Abstract

Single-crystal X-ray crystal-structure refinements and electron-microprobe analyses of two julgoldite samples, in one case complemented by Mossbauer spectroscopy to determine the Fe valence distribution, yielded the compositions Ca2.00X(Fe2+0.44Fe3+0.29Mg0.17Al0.10)Y(Fe3+0.82Al0.18)2Si3O10.39(OH)3.61 (Bombay) and Ca2.00X(Fe0.67Mg0.16Al0.17)Y(Fe3+0.81Al0.19)2Si3O14-n(OH)n (4 ≥ n ≥ 3) (Kreimbach/Kaulbach), respectively. Structure refinements also allowed specification of the system of hydrogen bonds, which concur with those known for pumpellyite. A review of structural and chemical data for pumpellyite-group minerals including six julgoldite samples indicated that the length of the b axis is governed by the average size of ionic radii at the Y site, except for one sample of julgoldite-(Fe3+). Size effects of the concentration of “large” divalent cations (Mg, Fe) at the X site can be seen in the lateral extension of the (010) plane that acts as a buffer to accomplish the variable size of the X octahedra. Using unit-cell plots, the data suggest that five of the six tested julgoldite sample are julgoldite-(Fe2+) and only the one sample of the Bombay area studied by Artioli et al. (2003) is julgoldite-(Fe3+). Both samples studied in this paper were classified into julgoldite-(Fe2+). In contrast to pumpellyite samples sensu stricto, which exhibit a Me2+:Me3+ ratio of approximately 1:1, julgoldite samples may cover the whole range between Ca2X(Fe3+)Y(Fe3+)2Si3O11(OH)3 and Ca2X(Fe2+)Y(Fe3+)2Si3O10(OH)4.