Stability and Relations of the Al-Fe Epidotes

R. G. J. Strens
Department of Geology and Geophysics, University of California, Berkeley1
1Now at the Department of Mineralogy and Petrology, Downing Place, Cambridge.

Summary: The various (P, T) stability fields of iron-free zoisite have been deduced for systems containing excess silica and water with Ca : Al ratios ranging from 1 : 2 (anorthite) to 3:2 (grossular). Zoisite is a possible phase in all systems with Ca:Al lower than 3:2, attaining its maximum stability at the zoisite (Ca:Al = 2:3) and prehnite (Ca:Al = 2:2) compositions.

The consequences of varying the Al : Fe ratio are next examined. Zoisite with 4 % pistacite is stable to 525° at 2 kilobars, compared with ∼585° for clinozoisite and 620 to 630° for epidote (Ps35) at the same pressure. Increasing iron content also stabilizes epidote minerals relative to their low-temperature and high-pressure decomposition products.

Examples of natural zoisite-clinozoisite, zoisite-epidote, and clinozoisite-epidote assemblages are described. It is concluded that zoisite-epidote mixtures result from the disproportionation of clinozoisite outside its own stability field, but within those of zoisite and epidote. The assemblage zoisite-clinozoisite is probably not stable, but further evidence is needed on this point. The assemblage clinozoisite-epidote is stable below 550° C, at which temperature the solvus in the Al-Fe series closes.

Mineralogical Magazine; September 1965 v. 35; no. 271; p. 464-475; DOI: 10.1180/minmag.1965.035.271.02
© 1965, The Mineralogical Society
Mineralogical Society (