February 1, 2012, Chantilly, Virginia:

Mills et al. (p. 415) report the first confirmed case of twinning by merohedry observed both morphologically and structurally in a mineral of the apatite supergroup, namely pyromorphite. Twinning is the oriented association of two or more individuals that are related by a twin operation belonging to the point group either of the lattice (twinning by merohedry) or of a sublattice (twinning by reticular merohedry), but not to the point group of the individual.

The mechanisms by which trace/minor elements can enter the crystal structures of sulfide minerals are of both scientific and economic interest. Economic interest is generated when enrichment in certain trace elements increase the commercial value of mined concentrates of common sulfides. Indium-bearing sphalerite is one such example. Indium enters sphalerite together with copper. The study by Cook et al. (p. 476) shows that in indium-substituted sphalerite, Cu is present in the univalent state. Measurements were made by Synchtroron Cu K edge micro-XANES measurements on foild prepared in situ by FIB-SEM methods. The results offer indirect proof for the coupled substitution 2 Zn2+ <-> Cu+ + In3+, which allows indium to enter the sphalerite structure. Moreover, the study clearly demonstrates the utility of synchrotron radiation to accurately determine oxidation state in small volumes of mineral in which the concentration of the element of interest is low or very low.

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