Hydrothermal syntheses of epidote and piemontites on the join Ca₂Al₂Fe³⁺Si₃O₁₂(OH)-Ca₂Al₂Mn³⁺Si₃O₁₂(OH) at relatively low pressures of 200-400 MPa

Abstract

Syntheses of epidote minerals on the join Ca₂Al₂Fe³⁺Si₃O₁₂(OH)-Ca₂Al₂Mn³⁺Si₃O₁₂(OH) were conducted using standard cold-seal pressure vessels and solid oxygen buffering techniques at 200-400 MPa and 500 °C. Two types of starting materials were used: (1) oxide mixtures (type 1) + excess H₂O, and (2) mixtures of synthetic CaFe³⁺AlSiO₆-pyroxene and oxides (type 2) + excess H₂O. The compositions of Ca₂Al₂Fe³⁺_qMn³⁺_1-qSi₃O_12.5 starting materials were q = 1.0, 0.9, 0.5 and 0.25 for the type 1, and q = 1.0, 0.75, 0.5 and 0.25 for the type 2. Mn³⁺-free epidote was not crystallized from the type 1 starting materials, even at 400 MPa, whereas Mn³⁺-Fe³⁺-Al piemontites were produced at 300 MPa, but they were along with some associated minerals. Recrystallization of hematite during runs is a major problem when using oxide mixture starting materials. However, recrystallization of hematite was suppressed by using the type 2 starting materials, and Mn-free epidote and Mn³⁺-Fe³⁺-Al piemontite were synthesized as almost single phases at 380 MPa and at 200-370 MPa, respectively. The chemical compositions of the synthetic Mn³⁺-free epidote and Mn³⁺-Fe³⁺-Al piemontites formed were generally close to those of the starting materials, except that Fe³⁺ contents tended to be slightly lower.
     Pressure, temperature and oxygen fugacity have been considered to be the main factors controlling the occurrence of epidote minerals. However, this study demonstrates the importance of Fe³⁺-source material. Although the results of previous synthesis experiments of epidote minerals have not necessarily been compatible with those of stability experiments and the natural occurrences of epidote minerals, the syntheses here succeeded in producing nearly single phase epidotes with Ca₂Al₂(Fe³⁺,Mn³⁺)Si₃O₁₂(OH) at relatively low pressures less than 400 MPa. This aids the understanding of the varied occurrence of natural epidote minerals.