Abstract

Investigation of 12 natural sodic amphiboles, all but one of which crystallized under low-grade metamorphic conditions, confirms the conclusions of earlier workers that re-fringence and unit cell dimensions increase with increasing iron content. The present study further demonstrates that Fe3+ is concentrated in the M(2) structural position whereas Fe2+ is ordered in M(l) and M(3). Aluminous sodic amphiboles characteristically display a Fe2+/Mg fractionation where M(3) is enriched in ferrous iron relative to M(1); the converse relationship seems to hold for iron-rich analogues.

Heat treatment of iron-bearing sodic amphiboles in air at 705±2°C results in rapid loss of hydrogen and concomitant increase in ferric: ferrous ratio. To the extent that Fe2+ ions are available in M(1) and M(3) sites, electrons are transferred to adjacent bonded hydroxyls, allowing neutralization and expulsion of hydrogen. Dehydrogenation takes place over a period on the order of an hour under the experimental conditions. Subsequent to loss of all hydrogen, continued oxidation takes place by a much slower process, possibly involving electron and/or ion diffusion. In addition, significant disordering of cations occurs on experiments of four days’ duration. Sodic oxyamphiboles possess shorter a, b and c axis repeats, higher indices of refraction and birefringence compared to natural starting materials.

Natural iron-bearing glaucophane hydrothermally heated for 15,667 hours at 513 ± 10°C, Pfluid of 2000 bars and fo2 of about 10-22 bars, was partially oxidized, completely lost its hydrogen (as H2O?), and now displays both unusually large a and b axis dimensions and elevated refringence. It is tentatively concluded that the octahedrally coordinated cations are virtually completely disordered in this oxyglaucophane anhydride.

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.