PHASE RELATIONS OF RIEBECKITE-ARFVEDSONITE SOLID SOLUTIONS

Terminal breakdown reactions of riebeckite on the HM and FMQ buffers are O₂ conserved, respectively: (1) Rbk + 3 Hem=2 Aeg + 3 Mag + 4 Qtz + H₂O and (2) 2 Rbk=4 Aeg + 3 Fa + 5 Qtz + 2 H₂O. These differ by reaction (3): 2 Mag + Qtz=Fa + 2 Hem. The large Gibbs free energy of reaction 3 causes a roughly 300^oC separation between the PT-curves for 1 and 2. This results in a positive slope in the isobaric logfO₂-T diagram for the univariant reaction (4): 2 Rbk + O₂=4 Aeg + 2 Mag + 8 Qtz + 2 H₂O which links the points for 1 and 2.

Experiments on a natural crocidolite (89% Rbk 11% Mg-Rbk) by Owen (1988, thesis) show that reaction 1 in the pure system NFSHO is below 367^oC at 2 kbar. Reaction 2 for pure riebeckite is metastable with respect to riebeckite-arfvedsonite solid solution, as shown experimentally by Ernst (1962) and Owen (1988). Along a path of increasing temperature parallel to the HM and FMQ buffer curves, riebeckite becomes more arfvedsonitic because of reaction (5): 18 Rbk=12 Arf + 10 Mag + 48 Qtz + O₂ + 6 H₂O. Below the FMQ buffer curve the analogous reaction is (6): 6 Rbk=4 Arf + 5 Fa + 11 Qtz + 2 O₂ + 2 H₂O. Curves 5, 6 and 7 (6 Rbk + 24 Aeg + 2 Mag + 6 H₂O=12 Arf + 7 O₂) limit the stability of a given Rbk-Arf solid solution to a thin wedge-shaped region on the logfO₂–T diagram. Maximal thermal stability on FMQ is about 760^oC at 2 kbar where the composition is roughly 60% of the Arf component (Owen 1988). This is why the sodic amphibole in sodic granites is arfvedsonitic rather than riebeckitic. Arf-rich amphibole is possible at very low to high temperatures below FMQ. As much as 10% grunerite component may be present in metamorphic Rbk-Arf, depending on temperature and fO₂.