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.
