Determanation of Fe³⁺ and Fe²⁺ Concentrations in Feldspar by Optical Absorption and EPR Spectroscopy

Anne M. Hofmeister and George R. Rossman
Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.

ABSTRACT

Ferrous and ferric iron contencentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ± 15 percent of the amount present. Optical spectra indicate that Fe²⁺ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe²⁺/Fe total ration on An content demonstrates that Fe²⁺ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe³⁺ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-itegrated intensities with optical peak areas shows that all Fe³⁺ is tetrahedrally coordinated in both plkagioclase and isordered potassijum feldspar. Microcline perthites show, in addition to tetrahedral Fe³⁺, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions.