- © The Mineralogical Society Of America
This chapter is concerned with in situ studies of the structural behavior of amphiboles under non-ambient conditions. Relatively few such studies have been made compared with those on samples studied under ambient conditions (room-P and T), which form the staple of many spectroscopic and diffraction studies of short-range and long-range order.
Collecting data under non-ambient conditions often imposes significant additional complexities in the data-collection and data-handling procedures than are necessary in ambient studies. For example, collecting high-pressure X-ray data for a sample in a diamond-anvil cell (DAC) usually means a much-reduced dataset due to limited access to reciprocal space (about 40%) imposed by the pressure cell. In order to get good coverage of reciprocal space, it is sometimes necessary to run two crystals of the same sample cut and mounted in the DAC in different orientations. The presence of pressure or temperature assemblies (DACs, multi-anvil devices, cryostats, furnaces) also requires the application of high-quality background corrections. Suffice it to say that many non-ambient in situ experiments are not trivial to perform and, in some cases, they require access to national facilities (e.g., synchrotron and neutron sources). For these reasons, non-ambient in situ studies are much less common than “quench-and-look” approaches (particularly spectroscopy), which tend to focus upon compositional systematics. However, the importance of in situ studies of non-ambient behavior cannot be overstated. Many materials undergo non-quenchable displacive phase transitions at high pressure and high temperature, and these transitions can produce microtextures and be associated with anomalous thermodynamic behavior. The determination of compressibilities and expansivities provides fundamental thermodynamic data and allows the behavioral trends to be identified between structurally and compositionally related phases.
It is well-known that monoclinic amphiboles with significant Mg occupancy of M(4) undergo a reversible displacive phase transition involving P21/m and C …