Abstract
The mechanisms and kinetics of the breakdown reactions of anorthite to zoisite, clinozoisite, and other products in unseeded runs have been studied at 400–700°C and 10–20 kbar, using X-ray diffractometry and scanning electron microscopy. Most of the runs were made at 10 kbar, and at this pressure, the equilibrium reactions in the anorthite–water portion of the CaO–Al2O3–SiO2–H2O system do not occur in the absence of stable reaction products, and are replaced by metastable reactions involving the formation of an aluminous mullite and zoisite and margarite:
At T ≤ 500°C and at higher pressures, clinozoisite develops metastably at the expense of zoisite. The incoming of (5) at 675°C coincides with metastable projection of the P–T curve for the equilibrium reaction (5). All reactions involve solution-precipitation processes, with nucleation and growth of products appearing to initiate heterogeneously, but not epitaxially, on anorthite surfaces. Partially reacted anorthite grains show evidence of crystallographically controlled dissolution. The reaction rate increases from 400°C to 500°C, but decreases from 600°C to 700°C as the anorthite ⇌ zoisite boundary is approached. The experimental data favor the dissolution of anorthite as being the critical rate-determining process, and are consistent with the preservation of plagioclase in granulites.
