Synthesis and stability relations of Mn-Al piemontite, Ca ₂ MnAl ₂ Si ₃ O ₁₂ (OH)

Stability relations for the Mn−Al piemontite bulk composition Ca₂MnAl₂Si₃O₁₂(OH) were investigated at 1 and 2 kbar and temperatures between 200° and 750°C, using cold-seal pressure apparatus and solid oxygen buffer techniques. Pure piemontite, with average cell dimensions a = 8.86(1), b = 5.69(1), с = 10.19(2)A and β = 115°42′, was readily synthesized from oxide mixtures at 500°−600°C and fO₂ defined by the Cu–Cu₂O and Cu₂O–CuO buffers at 1 and 2 kbar. The high-temperature equivalent assemblage is intermediate grossular-spessartine garnet solid solution [Ca₂MnAl₂Si₃O₁₂: a = 11.804(2)A, n_D = 1.7636(2)] + fluid for fO₂ defined by the QFM, HM, CC, and CT buffers. The cell parameters for synthetic piemontites and garnets grown over a range of fO₂ conditions show only small and random variations; this relation suggests that compositional changes of garnet and piemontite for the bulk composition Pm₃₃Cz₈₇ with variation of fO₂ are limited. Average measured a for the synthetic garnets is slightly higher than the calculated value for this bulk composition.

In agreement with evidence from natural piemontite-bearing parageneses, reversal runs indicate that crystallization of piemontite requires a high oxygen fugacity. Along the HM buffer, garnet was the only condensed phase stable at all experimental conditions. Equilibrium reversal for the reaction piemontite = garnet + fluid [Ca₂MnAl₂Si₃O₁₂(OH) = Ca₂MnAl₂Si₃O₁₂ + 1/2 H₂O + 1/4 O₂] was delineated, for 2 kbar at 617°±10° for the CT buffer, at 404°±10° for the CC buffer, and below 250°C for the HM buffer. At Pfluid = 1 kbar, piemontite is stable up to 591°±10° for the CT buffer and to 402°±10°C for the CC buffer. The effect of fluid pressure on the stability of piemontite is apparently minor compared to that of oxygen fugacity.

The sporadic occurrence of piemontite in a wide variety of geologic environments from blueschist to greenschist and amphibolite facies conditions is mainly controlled by oxygen fugacity in addition to pressure, temperature, and major-element composition of the host rocks. Introduction of Fe into piemontite in natural compositions will evidently result in a more complex breakdown reaction and in an extension of the piemontite stability field to higher temperatures and lower oxygen fugacities.