Abstract

Structural changes that accompany oxidation-dehydrogenation in amphiboles are examined using bond-length and bond-strength data from six crystal-structure refinements. These include tschermakitic hornblende (untreated; hydrothermally heated at 650 °C, 1 kbar, 4 d at the magnetite-hematite oxygen buffer; heated in air at 700 °C, 1 atm, 30 min) and literature data for riebeckite (untreated; heated in air at 650 °C, 1 atm, 4 d) and a natural potassian titanian magnesio-hastingsite that contains an appreciable oxy-amphibole component.

These data show that dehydrogenation results in dramatic decreases in bond strength at 0(3). About half of the total decrease is compensated by shortening of M-0(3) bond lengths and preferential ordering of trivalent cations at M(1) and M(3). The ordering involves not only the Fe3+ produced by oxidation but also trivalent cations originally residing at M(2) in the unoxidized structures.

At least part of the remaining loss of bond strength is offset by interaction of the A-site cation with O(3). In riebeckite, this involves migration of Na from M(4) to the previously vacant A site. In tschermakitic hornblende, there is a slight loss of cations, presumably Na, from M(4) in the air-heated sample. There is also shortening of the A(m)-O(3) distances in both heated structures and an increase in the A(m) occupancy of the air-heated sample, apparently at the expense of A(2/m) and M(4).

Previous investigators have found that in air-heated grunerite, unlike these amphiboles, dehydrogenation is not the only initial oxidation mechanism. This may result from the fact that the availability of compensation mechanisms is limited in grunerite.

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