Cavoite, CaV3O7, a new mineral from the Gambatesa mine, northern Apennines, Italy

Abstract

Cavoite, CaV 3 O 7 , occurs at the Gambatesa mine (Liguria, Italy) filling microcavities in massive bands of caryopilite + calcian rhodochrosite ± quartz. Cavoite has been found as very rare radiated aggregates of strongly elongated prismatic to acicular crystals up to about 0.28 mm in length, closely associated with an unidentified silicate phase. The crystals vary in colour from colourless to olive green-brown; they are brittle, transparent and non-fluorescent, with vitreous lustre and near white streak; no cleavage, parting or twinning were observed. The mean empirical formula from microprobe analyses, based on seven oxygen atoms, is (Ca 0.95 Mn 0.03 K 0.02 )(V 2.79 Si 0.22 )O 7 . It well approaches the ideal one, CaV 3 O 7 , taking into account that the chemical determinations (presence of K, Mn and Si) are possibly affected by contamination due to the close association with the unidentified silicate. The powder-diffraction data give the refined cell parameters a = 10.42(2) A, b = 5.28(2) A, c = 10.34(2) A and V = 568.2 A 3 in the space group Pnam . Micrometric crystals of cavoite were also investigated by means of transmission and analytical electron microscopy. Cavoite is the natural analogue of the synthetic CaV 3 O 7 , whose structural study is reported in literature together with that of the isostructural phases SrV 3 O 7 and CdV 3 O 7 .