M�ssbauer studies of synthetic and natural micas on the polylithionite-siderophyllite join

Abstract

Mossbauer studies of micas on the polylithionite-side-rophyllite join show the existence of a relation between the quadrupole splitting (ΔEQ) values of Fe2+ high spin doublets and both cationic and anionic composition of micas. This linear relation is positive as Li2O content increases and negative as F content increases. In the lithium iron micas, the inner ferrous quadrupole doublet is assigned to the cis-site M(2), while the outer doublet is assigned to the trans-site M(1). A random distribution of Fe2+ is observed in fluorine-rich compositions, while slight enrichment of the M(1) site is noticed in hydroxyl compositions, perhaps due to a more sensitive oxidation in situ in M(2) than M(1) sites. The Mossbauer spectrum of siderophyllite K2(Fe42+Al2)(Si4Al4)O20(OH)4 shows the presence of only one ferrous doublet, which is assigned to M(2) sites. Hence from Mossbauer data we must consider a clintonite (“xanthophyllite”) structure for this mica. The ordered octahedral layer has two distorted ferrous cis-sites and one, more symmetrical, aluminum trans-site.