A Mössbauer study of coexisting hornblendes and biotites: quantitative Fe3+/Fe2+ ratios

Room temperature ⁵⁷Fe Mössbauer spectra have been recorded for six hornblendes and seven biotites from the Sierra Nevada Batholith, California. The hornblende spectra have been resolved into three Fe²⁺ doublets due to Fe ²⁺ in M1, M2, and M3; and one Fe³⁺ doublet due to Fe³⁺ mainly in M2. The biotite spectra have been resolved into two Fe²⁺and two Fe³⁺ doublets due to Fe²⁺ and Fe³⁺ in both Ml and M2. Using the areas of the peaks, Fe³⁺/Fe²⁺ ratios have been derived. For biotites, these are found to be in good agreement with the chemical analysis values; for hornblendes, the constant relating the chemical and Mössbauer Fe³⁺ /Fe²⁺ values is 1.13 ± 0.02. The accuracy of the non-destructive Fe³⁺/Fe²⁺ analysis is within 2 to 4 percent for most samples. Saturation corrections for such analyses are shown to be negligible

The ferrous site populations show that Fe²⁺ orders in the Ml and M3 positions in hornblendes. There is a small amount of ferrous ordering on M2 in biotites, which increases as the ferrous content decreases.