NATURALLY-OCCURRING FERRIC IRON SANIDINE FROM THE LEUCITE HILLS LAMPROITE

Sanidine crystals precipitating from lamproitic magmas characteristica lly contain appreciable amounts of Fe3+ Sanidine grains from the Leuci te Hills lamproites (Wyoming) have compositions in which up to 70 mol% of the KAlSi3O8 molecule is replaced by KFe3+Si3O8, thus constituting a new natural feldspar component. Sanidine is among the last phases t o crystallize in the groundmass of Leucite Hills lamproites and rarely exceeds 150 mu m in length. Nearly all grains are zoned in Fe, rangin g from 2.5 wt% Fe2O3 in the cores to 19.6 wt% Fe2O3 at rims adjacent t o quenched glass. The Fe-rich rims are relatively narrow on (010) crys tal faces in comparison with (001) faces (maximum width 10 mu m), prob ably reflecting faster- (001) and slower-growing (0 10) crystallograph ic directions. A near-perfect correlation is obtained by combining Ti, Mg, and excess Si cations with Fe3+ in tetrahedral substitution for A l. Insufficient numbers of R(2+) cations exist to balance the high con centrations of Mg through an R(2+)Mg(2+)R(-1)(1+)R(-1)(3+) substitutio n, suggesting the exchange Mg2+Si4+Al-23+ in ZKAlSi(3)O(8) analogous t o that found in synthetic leucite and kalsilite. Selected-area electro n diffraction and convergent-beam electron diffraction show that the F e-rich sanidine rims have a C2/m structure with cell dimensions of a = 8.68 +/- 0.15, b = 13.14 +/- 0.23, c = 7.31 +/- 0.15 Angstrom, alpha = 90.0 +/- 0.54, beta = 116.0 +/- 0.46, gamma = 90.0 +/- 0.33 degrees, and a calculated cell volume of 747 +/- 21 Angstrom(3). These crystal lographic data are in very close agreement to those obtained previousl y for synthetic KFe3+Si3O8. The unusually high concentrations of Fe3+, Ti, and Mg in the sanidine rims are likely the consequence of rapid c rystallization during quenching of an unusually Si- and K-rich, yet Al -deficient residual magma.