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 reexamined using specimens from its type locality, Sjögruvan, Örebro 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; Mössbauer spectroscopy was used for the Swedish
... pecimen. Electron microprobe data indicate a close to ideal composition Mn2Fe 3+ Sb 5+ O6 for both samples, whereas Mössbauer 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 Sjögruvan are a = 5.2351(3), c = 14.3645(8) Å, V = 340.93(4) Å 3 , and a = 5.2314(2), c = 14.3518(8) Å, V = 340.15(3) Å 3 , respectively. Melanostibite is an homeotypic derivative of pyrophanite.