VOLASCHIOITE, Fe3+4(SO4)O2(OH)6{middle dot}2H2O, A NEW MINERAL SPECIES FROM FORNOVOLASCO, APUAN ALPS, TUSCANY, ITALY

Abstract

Volaschioite, ideally Fe 3+ 4 (SO 4 )O 2 (OH) 6 ·2H 2 O, occurs in a small magnetite–pyrite orebody near Fornovolasco, in the Apuan Alps, Tuscany, Italy. The new mineral species is associated with pyrite and iron oxyhydroxides. In the same occurrence, other iron sulfates were identified, including alum-(K), “copiapite”, fibroferrite, halotrichite, melanterite, romerite, and voltaite. Volaschioite occurs as radial aggregates of bladed crystals, up to 100 μm in length and less than 5 μm across. The color is yellowish orange with an orange streak; the luster is vitreous to resinous. The mineral is transparent and strongly pleochroic. Electron-microprobe analyses lead to the empirical formula Fe 4.16 (SO 4 ) 0.92 O 2.32 (OH) 6 ·2H 2 O; the calculated density is 3.03 g/cm 3 . Volaschioite is monoclinic, with a 16.068(4), b 3.058(1), c 10.929(2) A, β 93.82(3)°, V 535.8(2) A 3 , Z = 2, space group C 2/ m . The strongest three X-ray powder-diffraction lines [ d in A( I )( hkl )] are: 8.03(100)(200), 4.37(24)(202), 3.989(22)(400). The single-crystal X-ray diffraction pattern shows the presence of additional very weak and streaked reflections indicating a doubling of the b parameter. The average structure of volaschioite is composed of ribbons of edge-sharing Fe-centered octahedra running along b , linked together by corner-sharing to form corrugated layers, decorated on both sides by sulfate groups. Additional H 2 O molecules are located between the layers. Volaschioite is the product of pyrite alteration in an oxidizing environment. The name of this new species refers to the ancient name of the type locality, Forno de Volaschio . This mineral and its name have been approved by the IMA–CNMNC (IMA2010–005).