Abstract

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

 
Fe2++OH=Fe3++O2+½H2,

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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.