Stability of Epidote Minerals

Abstract

A NUMBER of silicate minerals have proved difficult to synthesize by the commonly employed methods involving hydrothermal crystallization of glasses or reactive oxide mixes of suitable composition. Failure to synthesize a phase may be caused by the experimental conditions not being within the region where the phase is thermodynamically stable. But, in some cases, failure simply reflects a large nucleation barrier to the formation of the stable phase so that metastable phases which nucleate with ease form, and may persist indefinitely. Several workers^1–3 have considered this problem in relation to the epidote minerals, and the consistent failure of low-pressure synthesis could indicate that these phases may be stable only under relatively high-pressure conditions. Geological occurrence does not entirely support this proposition. The results presented here indicate that with these minerals nucleation is a sluggish process at low pressures, and if this step is by-passed, growth is readily achieved from the phases which normally form in synthesis. This sluggish nucleation is possibly related to the structure of the minerals.