Synthesis of high-density evacuated hot-pressed olivine aggregates

Naturally derived, fine-grained olivine ceramics were synthesized by an evacuated hot-pressing method that yielded samples with porosities of < 0.1%, a marked reduction compared to samples fabricated by conventional hot pressing with porosities of ≳1%. Evacuated hot pressing yields bright-green olivine samples that are transparent at the mm-scale, resembling the appearance of the single crystals from which they were synthesized, while conventional hot pressing produces milky-green aggregates that are opaque. This contrast in macro-scale transparency reflects the difference in micro-scale porosity. Annealing experiments on samples synthesized by the two different methods, some at 1 atm and others at 300 MPa confining pressure, reveal contrasting styles of grain and pore growth. During high-temperature annealing at both low and high pressures, evacuated hot-pressed samples underwent rapid, abnormal grain growth, while conventionally hot-pressed samples remained fine grained over long annealing times with very limited abnormal grain growth. During annealing at 1 atm, evacuated hot-pressed samples exhibited very little pore growth compared to conventionally hot-pressed samples that disaggregated due to pervasive pore growth. These experiments demonstrate the primary influence that porosity plays in grain growth in olivine aggregates. Further, the methods presented in this study provide a means to produce low-porosity olivine aggregates from naturally derived powders that can be used for high-temperature experiments at low pressures, as well as a method to make dense, coarse-grained olivine aggregates for laboratory studies.