To investigate the relation between oxygen fugacity, ferric-ferrous ratio (R), and unitcell parameters, four natural clinoamphiboles (grunerite, tschermakitic hornblende, mag-nesio-homblende, and a riebeckite-arfvedsonite solid solution) were reacted at 650°C, 1 kbar at oxygen fugacities defined by several solid oxygen-buffer assemblages. In order to produce more highly oxidized samples, heating in air at 700 °C was also carried out. Variation in R is accomplished mainly by the oxidation-dehydrogenation equilibrium
but the results suggest that other mechanisms may also be involved. All four amphiboles exhibited systematically higher ferric-ferrous ratios with increasing fO2 of equilibration. Equilibrium R values were achieved relatively rapidly and could be readily restored to original values by treatment at the appropriate buffer. In some cases, a metastable equilibrium of ferric-ferrous ratio was achieved before the amphibole decomposed to other Fe3+-bearing phases.
Of the four amphiboles, grunerite is apparently the least able to accommodate Fe3+ within its crystal structure and decomposes at relatively higher oxygen fugacities.
The value of a sin β decreases systematically uniformly as the Fe3+ content of tschermakitic hornblende, magnesio-homblende, and riebeckite increases, reflecting increasing Fe3+ in the octahedral cation sites. The variation in a sin β of grunerite is significantly less than for the other three amphiboles. The variation in b for the two hornblendes suggests that Fe3+ produced by oxidation is not strongly ordered into the M(2) site.