GROWTH MECHANISMS, STRUCTURAL DEFECTS AND COMPOSITION OF SYNTHETIC TREMOLITE - WHAT ARE THE EFFECTS ON MACROSCOPIC PROPERTIES

Amorphous gels and oxides corresponding to Ca2Mg5Si8O23 in bulk compos ition have been reacted to phase assemblages containing tremolitic amp hibole using routine hydrothermal methods in the pressure/temperature range from 1-22 kbar and 600-875 degrees C. The products have been cha racterized by X-ray diffraction, optical microscopy and high-resolutio n electron microscopy. The nature of the amphibole microstructure and the types of intergrown biopyriboles vary markedly as a function of sy nthesis temperature and pressure, reflecting different amphibole growt h mechanisms. At P(H2O) < 10 kbar, within or very near the stability f ield of talc, amphibole forms by topotactic reaction from this metasta bly crystallized, intermediate phase; chain multiplicity faults (CMFs) with m greater than or equal to 3 are numerous; the bulk compositiona l shift in the disordered amphibole crystals is compensated in the run product by coexisting diopside. At P(H2O) greater than or equal to 10 kbar, but still in the talc stability field, amphibole grows topotact ically from and also nucleates preferentially on metastable diopside b y a dissolution/regrowth process; the two phases form fine (approximat e to 100 Angstrom lamellar intergrowths with almost no CMFs with m gre ater than or equal to 3, and the compositional shift induced by armour ed diopside relies is compensated by discrete talc. The same lamellar amphibole/diopside microstructure is observed at temperatures above th e talc stability field and at all pressures investigated, but the comp ositional shift is compensated by enstatite + quartz. Varying the expe rimental parameters (run duration; H2O content) does not significantly affect the above observations. For the wide range of pressures and te mperatures investigated, the synthetic amphiboles of the present study appear to correspond very closely to end-member tremolite. The observ ed product assemblages and apparent compositional shifts of the amphib ole, equivalent in Ca/Mg ratio to as much as 11 mol% magnesiocummingto nite 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 si gnificant role for the ''10 mol% magnesiocummingtonite component'' com monly assumed in analogous experiments in the present literature. The genuine magnesiocummingtonite solid-solution component may be much les s than 10 mol%. Empirical high-resolution transmission electron micros copy observations of preferential corrosion structures at crystal term inations suggest that, to a first approximation, structurally disorder ed tremolites exhibit the reaction behaviour of mechanical mixtures do wn to the unit-cell scale. The thermodynamic properties of synthetic t remolite, 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 rec ent comparative experimental studies on the phase relationships of nat ural and synthetic tremolite.