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-qSi3O12.5 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.