Fe-free clinozoisite stability relative to zoisite

Abstract

Orthorhombic zoisite, Ca₂Al₃Si₃O₁₂OH, and its monoclinic polymorph, clinozoisite, are key indicator minerals in low- and middle-grade basic and calcareous metamorphic rocks, and often occur together in apparent equilibrium¹. Both minerals contain variable amounts of Fe³⁺ replacing Al, but clinozoisite always contains more Fe³⁺, with as much as 30–40% substitution in typical epidote. Interpretations of their relative stabilities, based on field relations^1–3 and experimental studies^4,5, have conflicted markedly. No unambiguous experimental demonstrations of reversibility of a zoisite–clinozoisite equilibrium have yet been performed, and it may well be that such an equilibrium relation, if it exists, lies in a portion of the P, T plane which is inaccessible to direct investigation. We present here experimental reversals of the Fe-free equilibrium: 2Ca₂Al₃Si₃O₁₂OH + Al₂SiO₅ + SiO₂ = 4CaAl₂Si₂O₈ + H₂O (1) zoisite or clinozoisite kyanite quartz anorthite using both clinozoisite and zoisite. At 550 °C, the reaction with zoisite is in equilibirum at 6.30±0.25 kbar and the reaction with clinozoisite is in equilibrium at 7.30±0.15 kbar. Therefore, clinozoisite is considerably less stable than zoisite at this temperature, and the reaction with clinozoisite, though reversible, is metastable. Reliable thermodynamic data for the phases show, with the present experimental data, that zoisite has lower Gibbs free energy at 550 °C by 1,020±400 cal mol⁻¹. Published composition data of natural coexisting zoisite and clinozoisite, together with first-order assumptions as to the equilibrium temperatures of the parageneses, suggest that zoisite entropy is ∼2.5 cal mol⁻¹ K⁻¹ higher than that of clinozoisite and that Fe-free clinozoisite becomes stable relative to zoisite below about 200 °C.