Bilbungsbedingungen von Epidot und Orthozoisit

Abstract

The equilibrium curves for the reactions:

• (a) 4 orthozoisite + 1 quartz ⇌ 5 anorthite + 1 grossularite + 2 H₂O.

• (b) epidote (+ quartz) ⇌ anorthite + grossularite/andradite + haematite + H₂O (+ quartz) have been worked out.

Both reactions were reversed in the presence of seeds. Without seeds no stable equilibrium could be achieved, because of the sluggish nucleation of epidote and orthozoisite. From the starting oxide mixtures anorthite and garnet have been formed at all experimental conditions, and from these epidote or orthozoisite, respectively, have been crystallized in their stability fields. The experiments showed, that both epidote and orthozoisite can form at lower pressures than have hitherto been repor ted. Epidote will form at H₂O-pressures 1000–2000 bars lower (at temperatures up to 600° C) than are needed for the formation of orthozoisite. This explains, why epidotes are observed much more frequently than orthozoisite in the outer zone of contact-metamorphism. Other petrographic observations like the presence of orthozoisite and the absence of epidote in some high grade metamorphic rocks can be well understood from the different course of the respective equilibrium curves.

Some equilibrium data for the orthozoisite- and the epidotereaction, respectively, are given below: