Mössbauer studies of natural olivines

Abstract

Mössbauer spectra of six natural olivines and their annealed equivalents have been measured at several temperatures up to 426°C. Four of the unannealed olivines of plutonic or metamorphic origin exhibit relative enrichment of Fe²⁺ in M1 whereas two olivines of volcanic origin exhibit preferential ordering of Fe²⁺ in M2. The quadrupole splitting (Δ) of Fe²⁺ in the M1 site is less than that of Fe²⁺ in the M2 site. Δ of M1 and M2 decrease linearly as a function of absorber temperature (T), with d(Δ_M1)/dT more negative than d (Δ_M2)/dT. This result is similar to that obtained by Brown and Prewitt (1973) who found that distortion of the M1 octahedron increases at a faster rate than that of M2 with rising crystal temperature. Our data also indicate that Δ of the M1 and M2 sites decrease and increase, respectively, with increasing Fe²⁺ content of M1, suggesting increased distortion of the Ml octahedron with increasing Fe²⁺ content. Isomer shifts (δ) at both sites show similar decreases with increasing absorber temperature and are not significantly affected by changes in Fe²⁺ content at either site. The slightly larger δ of the M2 site suggests that M2-O bonds are more ionic than M1-O bonds. The results of annealing experiments indicate that the Mg/Fe exchange reaction between the Ml and M2 sites is not as sluggish as previously reported by Virgo et al. (1972).