5:15 PM - 6:45 PM
[SCG44-P05] Influence of grain size and stress on the phase transformation of enstatite
Keywords:Enstatite, Phase transformation, grain size, stress
Phase transformation of minerals changes their physical and chemical properties. Based on the phase diagram, phase transformation occurs as a function of thermodynamic conditions such as temperature (T) and pressure (P). However, recent mineralogical studies have suggested that the stability of the mineral phase can be changed by its grain size and stress. In this study, we conducted annealing and deformation experiments using enstatite crystal aggregates, and then showed that the stability of the metastable enstatite changes with grain size and stress.
Enstatite (Mg2Si2O6) has three mineral phases at P = 0.1 MPa: protoenstatite (PEn) is stable at high temperatures (T = 985-1557°C), orthoenstatite (OEn) is stable at moderate temperatures (T = 500-985°C), and clinoenstatite (CEn) is stable at low temperatures (T < 500°C). To investigate the effects of grain size on the phase transformation of enstatite, the samples with different grain sizes were prepared by varying the sintering time from 1 to 50 hours at T = 1310 to 1360°C and P = 0.1 MPa. To understand the effects of the stress on the phase transformation, compressional deformation experiments with different stresses were performed at T = 1310°C and P = 0.1 MPa. After the experiments, the samples were analyzed by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). For the samples with a sintering temperature of 1360°C, they exploded after the sintering due to the volume change during the phase transformation from PEn to CEn. For the samples with a sintering temperature of 1310°C and 1345°C, they have metastable high temperature Pen and low temperature CEn crystals coexisting together. XRD and EBSD analyses of the samples with different grain size (~0.5 to 5 mm) showed that the proportion of mineral phases of CEn and PEn changes with their grain size, such as the CEn increases in the samples with larger grain size. These features indicate that the stability of metastable high-temperature PEn at room P-T conditions changes with grain size. The deformation experiments demonstrated that the samples were deformed under diffusion creep and no changes in the mineral fraction were observed in the deformed and undeformed samples. In contrast, the fraction of CEn phase in the samples increased due to cutting and polishing of the samples after the experiments. Therefore, the stability of the metastable PEn can be changed depending on the type of deformation, that is, either brittle or ductile. The EBSD analysis suggested that the CEn crystals have twining structure with a misorientation angle of 180° and a rotation axis of either [100] or [001]. These features imply that the phase transformation from PEn to CEn occurred by rotation of the unit cell and subsequently formed the twinning structure (transformation twin) within the CEn crystals. The number of defects that become nucleation cores for the phase transformation would be increased within a grain in the samples with larger grain size. Therefore, the stability of metastable high-emperature PEn also changes with grain size. The experimental results in this study imply that the stability of the metastable high-temperature PEn can be changed by its grain size and stress, which would be applicable not only to enstatite but also to other silicate minerals.
Enstatite (Mg2Si2O6) has three mineral phases at P = 0.1 MPa: protoenstatite (PEn) is stable at high temperatures (T = 985-1557°C), orthoenstatite (OEn) is stable at moderate temperatures (T = 500-985°C), and clinoenstatite (CEn) is stable at low temperatures (T < 500°C). To investigate the effects of grain size on the phase transformation of enstatite, the samples with different grain sizes were prepared by varying the sintering time from 1 to 50 hours at T = 1310 to 1360°C and P = 0.1 MPa. To understand the effects of the stress on the phase transformation, compressional deformation experiments with different stresses were performed at T = 1310°C and P = 0.1 MPa. After the experiments, the samples were analyzed by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). For the samples with a sintering temperature of 1360°C, they exploded after the sintering due to the volume change during the phase transformation from PEn to CEn. For the samples with a sintering temperature of 1310°C and 1345°C, they have metastable high temperature Pen and low temperature CEn crystals coexisting together. XRD and EBSD analyses of the samples with different grain size (~0.5 to 5 mm) showed that the proportion of mineral phases of CEn and PEn changes with their grain size, such as the CEn increases in the samples with larger grain size. These features indicate that the stability of metastable high-temperature PEn at room P-T conditions changes with grain size. The deformation experiments demonstrated that the samples were deformed under diffusion creep and no changes in the mineral fraction were observed in the deformed and undeformed samples. In contrast, the fraction of CEn phase in the samples increased due to cutting and polishing of the samples after the experiments. Therefore, the stability of the metastable PEn can be changed depending on the type of deformation, that is, either brittle or ductile. The EBSD analysis suggested that the CEn crystals have twining structure with a misorientation angle of 180° and a rotation axis of either [100] or [001]. These features imply that the phase transformation from PEn to CEn occurred by rotation of the unit cell and subsequently formed the twinning structure (transformation twin) within the CEn crystals. The number of defects that become nucleation cores for the phase transformation would be increased within a grain in the samples with larger grain size. Therefore, the stability of metastable high-emperature PEn also changes with grain size. The experimental results in this study imply that the stability of the metastable high-temperature PEn can be changed by its grain size and stress, which would be applicable not only to enstatite but also to other silicate minerals.