THE CRYSTAL-STRUCTURE AND CRYSTAL-CHEMISTRY OF FERNANDINITE AND CORVUSITE

Using type material of fernandinite (Ca,Na,K)xV8O20.4H2O) from Minasra gra, Peru, and corvusite (NaCa,K)xV8O20.4H2O) from the Jack Claim, La Sal Mountains, Utah, the properties and crystal chemistry of these min erals have been determined by Rietveld analysis of the powder X-ray-di ffraction patterns. The associated unit-cell parameters in space group C2/m (monoclinic) for fernandinite [corvusite] are: a 11.680(2) [11.7 06(4)] angstrom, b 3.6537(4) [3.644(1)] angstrom, c 11.023(2) [11.10(1 )] angstrom, beta 105.00(2) [103.46(7)]degrees. The crystal structure of both species is isotypic with the V2O5-type layer first found for d elta-Ag0.68V2O5; it consists of chains of VO6 octahedra linked by oppo site comers (parallel to b) condensed by edge-sharing to form the laye r. The vanadium has average valence 4.8, and the resulting layer-charg e is balanced by varying amounts of Ca, Na, and K in the interlayer re gion (x in the above formulas varies from 0.9 to 1.2), accompanied by labile water (amount depending on humidity and heat). This study has c onfirmed the validity of fernandinite as a unique mineral species. It is closely related to corvusite, from which it is distinguished on the basis of the dominant interlayer cation: Ca for fernandinite, Na for corvusite.