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.