Growth mechanisms, structural defects and composition of synthetic tremolite: what are the effects on macroscopic properties?

Abstract

Amorphous gels and oxides corresponding to Ca2Mg5Si8O23 in bulk composition have been reacted to phase assemblages containing tremolitic amphibole using routine hydrothermal methods in the pressure/temperature range from 1–22 kbar and 600–875° C. The products have been characterized by X-ray diffraction, optical microscopy and high-resolution electron microscopy. The nature of the amphibole microstructure and the types of intergrown biopyriboles vary markedly as a function of synthesis temperature and pressure, reflecting different amphibole growth mechanisms. At P(H2O)<10 kbar, within or very near the stability field of fale, amphibole forms by topotactic reaction from this metastably crystallized, intermediate phase; chain multiplicity faults (CMFs) with m≥3 are numerous; the bulk compositional shift in the disordered amphibole crystals is compensated in the run product by coexisting diopside. At P(H2O)≥10 kbar, but still in the talc stability field, amphibole grows topotactically from and also nucleates preferentially on metastable diopside by a dissolution/regrowth process; the two phases form fine (≈ 100 A) lamellar intergrowths with almost no CMFs with m≥3, and the compositional shift induced by armoured diopside relics is compensated by discrete talc. The same lamellar amphibole/diopside microstructure is observed at temperatures above the talc stability field and at all pressures investigated, but the compositional shift is compensated by enstatite+quartz. Varying the experimental parameters (run duration; H2O content) does not significantly affect the above observations. For the wide range of pressures and temperatures investigated, the synthetic amphiboles of the present study appear to correspond very closely to end-member tremolite. The observed product assemblages and apparent compositional shifts of the amphibole, equivalent in Ca/Mg ratio to as much as 11 mol% magnesiocummingtonite component, can be explained by the incorporation of CMFs in this phase. We argue that CMF-induced shifts in Ca/Mg ratios also play a significant role for the “10 mol% magnesiocummingtonite component” commonly assumed in analogous experiments in the present literature. The genuine magnesiocummingtonite solid-solution component may be much less than 10 mol%. Empirical high-resolution transmission electron microscopy observations of preferential corrosion structures at crystal terminations suggest that, to a first approximation, structurally disordered tremolites exhibit the reaction behaviour of mechanical mixtures down to the unit-cell scale. The thermodynamic properties of synthetic tremolite, even in small intergrown lamellae within disordered crystals, should therefore closely approach those of discrete, ideal tremolite single crystals, in accord with the converging agreement shown by recent comparative experimental studies on the phase relationships of natural and synthetic tremolite.