The amphiboles occurring in amphibolites and amphibole eclogites from the Sesia-Lanzo Zone have been studied with single-crystal X-ray diffractometry and electron microprobe analysis. The crystal chemical evolution from brown pre-Alpine hornblende to blue-green amphibole equilibrated under eclogite conditions has been followed in detail and interpreted in terms of P,T changes and bulk chemical control. The main compositional changes induced by increasing pressure from ~ 7 to ~ 15 kbar and decreasing temperature from ~ 700 to ~ 550 °C, are the expulsion of Ti from the octahedral M2 site and the transformation of Al(IV) into Al(VI), balanced by Na moving from the A to the M4 site, until ~ 0.5 Al(TV) and ~ 0.25 Na atoms p.f.u. in the A site are reached. The Na-occupancy at the M4 site and the trivalent octahedral cation contents are controlled by bulk rock composition. In completely re-equilibrated rocks, lacking any pre-Alpine relics, glaucophane or an almost colourless calcic-sodic amphibole, again characterized by ~ 0.5 Al(IV) and ~ 0.25 Na at the A site, has been found. These two compositionally-different amphiboles are compatible with the P,T conditions estimated for the Sesia-Lanzo eclogite rocks and plot on opposite sides of a compositional gap, which is related to the sum of the formal charges given by the site-population of (A + M4). A comparison is made with sodic and sodic-calcic amphiboles from other eclogites recently studied by single-crystal X-ray diffractometry.
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Ungaretti et al. (1983) studied this question.