New occurrences of derbylite, Fe2+xFe3+4-2xTi4+3+xSb3+O13(OH), and graeserite, Fe2+xFe3+4-2xTi4+3+xAs3+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 to0.4 apfu). Mössbauer spectroscopy confirmed the occurrence of Fe 2+ (up to 0.73 apfu), along with Fe 3+ . The Sb/(As+Sb) atomic ratio range between 0.73 and 0.82. Minor Ba and Pb (up to 0.04 apfu) occur. Derbylite is monoclinic, space group P21/m, with unit-cell parameters a 7.1690(3), b 14.3515(7), c 4.9867(2) Å, β 104.820(3)°, V 495.99(4) Å 3 . The crystal structure was refined to R1 = 0.0352 for 1955 reflections with Fo > 4σ(Fo). Graeserite occurs as prismatic to tabular black crystals, usually twinned, in carbonate veins or as as porphyroblasts in schist. Graeserite in the first kind of assemblage is V-rich (up to 0.66 apfu), whereas it is V-poor in the second one (0.03 apfu). Along with minor Cr (up to 0.06 apfu), this element replaces Fe and Ti. The occurrence of Fe 2+ (up to 0.68 apfu) is confirmed by Mössbauer 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. Unit-cell parameters are a 5.0225(7), b 14.3114(18), c 7.1743(9) Å, β 104.878(3)°, V 498.39(11) Å 3 and a 5.0275(4), b 14.2668(11), c 7.1663(5) Å, β 105.123(4)°, V 496.21(7) Å3 . The crystal structures of two graeserite samples were refined to R1 = 0.0399 and 0.0237 for 428 and 1081 reflections with Fo > 4σ(Fo), respectively. Derbylite and graeserite have homeotypic relations. They share the same tunnel structure, characterized by an octahedral framework and cuboctahedral cavities, hosting (As/Sb)O3 groups and (Ba/Pb) atoms
Derbylite and graeserite from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy): occurrence and crystal-chemistry
Biagioni, Cristian;Bonaccorsi, Elena;Perchiazzi, Natale;
2020-01-01
Abstract
New occurrences of derbylite, Fe2+xFe3+4-2xTi4+3+xSb3+O13(OH), and graeserite, Fe2+xFe3+4-2xTi4+3+xAs3+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 to0.4 apfu). Mössbauer spectroscopy confirmed the occurrence of Fe 2+ (up to 0.73 apfu), along with Fe 3+ . The Sb/(As+Sb) atomic ratio range between 0.73 and 0.82. Minor Ba and Pb (up to 0.04 apfu) occur. Derbylite is monoclinic, space group P21/m, with unit-cell parameters a 7.1690(3), b 14.3515(7), c 4.9867(2) Å, β 104.820(3)°, V 495.99(4) Å 3 . The crystal structure was refined to R1 = 0.0352 for 1955 reflections with Fo > 4σ(Fo). Graeserite occurs as prismatic to tabular black crystals, usually twinned, in carbonate veins or as as porphyroblasts in schist. Graeserite in the first kind of assemblage is V-rich (up to 0.66 apfu), whereas it is V-poor in the second one (0.03 apfu). Along with minor Cr (up to 0.06 apfu), this element replaces Fe and Ti. The occurrence of Fe 2+ (up to 0.68 apfu) is confirmed by Mössbauer 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. Unit-cell parameters are a 5.0225(7), b 14.3114(18), c 7.1743(9) Å, β 104.878(3)°, V 498.39(11) Å 3 and a 5.0275(4), b 14.2668(11), c 7.1663(5) Å, β 105.123(4)°, V 496.21(7) Å3 . The crystal structures of two graeserite samples were refined to R1 = 0.0399 and 0.0237 for 428 and 1081 reflections with Fo > 4σ(Fo), respectively. Derbylite and graeserite have homeotypic relations. They share the same tunnel structure, characterized by an octahedral framework and cuboctahedral cavities, hosting (As/Sb)O3 groups and (Ba/Pb) atomsI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
