Abstract
It is difficult to decipher the phase equilibrium of synthetic amphibole at low temperature, because of the sluggishness of reaction. The phase equilibrium is recomfirmed by the reversal run and the uniform composition of the run product. However, some problems remain, even if the experiments are considered to be close to representing equilibrium. One of problems is that Na and Si are dissoved in the coexisting water under the higher pressure than 10 kbar. Consequently, the bulk composition of synthetic amphibole must become nonstoichiometric, and the amphibole coexists with other phases in the field where amphibole alone should be a stable phase. The stability field of amphibole against temperature becomes smaller with increasing water content. For example, the composition of the synthetic pargasite shifts to the tremolite composition. Another problem is that the triple chain silicate is contained in the amphibole synthesized at low pressure (∼3 kbar) and low temperature (∼800°C) under excess water condition. The triple chain bearing amphibole might be confused with amphiboles, if the electron microprobe analyses were considered alone. It is also difficult to distinguish a few triple chain bearing amphibole from amphibole by X-ray powder diffraction patterns. Nevertheless, the stability and the phase relationship of the triple chain bearing amphibole are still not well understood. Further evidence for the triple chain bearing amphibole will be verified by high resolution electron microscope (HREM) and the experimental study.
