Derbylite and graeserite from the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy: occurrence and crystal-chemistry

New occurrences of derbylite, Fex2+Fe4-2x3+Ti3+x4+Sb3+O13(OH), and graeserite, Fex2+Fe4-2x3+Ti3+x4+As3+O13(OH), have been identified in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. Derbylite occurs as prismatic to acicular black crystals in carbonate veins. Iron and Ti are replaced by V (up to 0.29 atoms per formula unit, apfu) and minor Cr (up to 0.04 apfu). Mossbauer spectroscopy confirmed the occurrence of Fe2+ (up to 0.73 apfu), along with Fe3+. The Sb/(As+Sb) atomic ratio ranges between 0.73 and 0.82. Minor Ba and Pb (up to 0.04 apfu) substitute. Derbylite is monoclinic, space group P2(1)/m, with unit-cell parameters a= 7.1690(3), b = 14.3515(7), c= 4.9867(2) angstrom, beta = 104.820(3)degrees and V= 495.99(4) angstrom(3). The crystal structure was refined to R-1 = 0.0352 for 1955 reflections with > F-o > 4 sigma(F-o). Graeserite occurs as prismatic to tabular black crystals, usually twinned, in carbonate veins or as porphyroblasts in schist. Graeserite in the first kind of assemblage is V rich (up to 0.66 apfu), and V poor in the second kind (0.03 apfu). Along with minor Cr (up to 0.06 apfu), this element replaces Fe and Ti. The occurrence of Fe2+ (up to 0.68 apfu) is confirmed by Mossbauer spectroscopy. Arsenic is dominant over Sb and detectable amounts of Ba and Pb have been measured (up to 0.27 apfu). Graeserite is monoclinic, space group C2/m, with unit-cell parameters for two samples: a = 5.0225(7), b = 14.3114(18), c= 7.1743(9) angstrom, beta = 104.878(3)degrees, V = 498.39(11) angstrom(3); and a = 5.0275(4), b = 14.2668(11), c = 7.1663(5) angstrom, beta = 105.123(4)degrees and V= 496.21(7) angstrom(3). The crystal structures were refined to R-1 = 0.0399 and 0.0237 for 428 and 1081 reflections with F-o > 4 sigma(F-o), respectively. Derbylite and graeserite are homeotypic. They share the same tunnel structure, characterised by an octahedral framework and cuboctahedral cavities, hosting (As/Sb) O-3 groups and (Ba/Pb) atoms.