New data on melanostibite, Mn2Fe3+Sb5+O6

Following the identification of a new occurrence of melanostibite from the Apuan Alps, the crystal chemistry of this mineral has been re-examined using specimens from its type locality, Sjogruvan, orebro County, Sweden, and from the new occurrence, the Scortico-Ravazzone Mn ore deposit, Apuan Alps, Tuscany, Italy. Both specimens were examined through electron microprobe analysis, micro-Raman spectroscopy and single-crystal X-ray diffraction data; Mossbauer spectroscopy was used for the Swedish specimen. Electron microprobe data indicate a close to ideal composition Mn2Fe3+Sb5+O6 for both samples, whereas Mossbauer spectroscopy confirmed the trivalent oxidation state of Fe. Single-crystal X-ray diffraction for the Swedish and Italian specimens points to the acentric nature of melanostibite, space group R3. Refined unit-cell parameters of melanostibite from Scortico-Ravazzone and Sjogruvan are a = 5.2351(3), c = 14.3645(8) angstrom, V = 340.93(4) angstrom(3), and a = 5.2314(2), c = 14.3518(8) angstrom, V = 340.15(3) angstrom(3), respectively. Melanostibite is an homeotypic derivative of pyrophanite.