Naturally occurring ferric iron sanidine from the Leucite Hills lamproite

摘要

Sanidine crystals precipitating from lamproitic magmas characteristically contain appreciable amounts of Fe~(3+). Sanidine grains from the Leucite Hills lamproites (Wyoming) have compositions in which up to 70 mol percent of the KAlSi_3O_8 molecule is replaced by KFe~(3+)Si_3O_8, thus constituting a new natural feldspar component. Sanidine is among the last phases to crystallize in the groundmass of Leucite Hills lamproites and rarely exceeds 150 MHI in length. Nearly all grains are zoned in Fe, ranging from 2.5 wt percent Fe_2O_3 in the cores to 19.6 wt percent Fe_2O_3 at rims adjacent to quenched glass. The Fe-rich rims are relatively narrow on (010) crystal faces in comparison with (001) faces (maximum width 10 nm), probably reflecting faster- (001) and slower-growing (010) crystallographic directions. A near-perfect correlation is obtained by combining Ti, Mg, and excess Si cations with Fe~(3+) in tetrahedral substitution for Al. Insufficient numbers of R~(2+) cations exist to balance the high concentrations of Mg through an R~(2+)Mg~(2+)R_(-1)~(1+)R_(-1)~(3+) substitution, suggesting the exchange Mg~(2+)Si~(4+)A1_(-1)~(3+) in 2KAlSi_3O_8, analogous to that found in synthetic leucite and kalsilite. Selected-area electron diffraction and convergent-beam electron diffraction show that the Fe-rich sanidine rims have a Cllm structure with cell dimensions of a = 8.68 + - 0.15, b = 13.14 + - 0.23, c = 7.31 + - 0.15 A, a = 90.0 + - 0.54, beta = 116.0 + - 0.46, 7 = 90.0 + - 0.33 deg, and a calculated cell volume of 747 + - 21 A~3. These crystallographic data are in very close agreement to those obtained previously for synthetic KFe~(3+)Si_3O_8. The unusually high concentrations of Fe~(3+), Ti, and Mg in the sanidine rims are likely the consequence of rapid crystallization during quenching of an unusually Si-and K-rich, yet Al-deficient residual magma.