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While the bulk properties of the Earth are dependent upon the structures and properties of the minerals present, abrupt changes in rock properties can result from the transformation of the constituent minerals into either new structures, or completely new mineral assemblages. The largest-scale examples of such transformations are the global seismic discontinuities at depths of 410 km and 660 km within the Earth, which are believed to be the result of transformations in the major mantle minerals (see 3.1). Phase transformations at much lower temperatures and pressures are also used on a smaller scale in petrology to map the temperatures and pressures experienced by rocks during their formation and subsequent metamorphism. Also, phase transitions, in the sense of more subtle structural changes in a mineral phase while retaining the same basic atomic structure, also give rise to significant changes in the physical properties.

We wish to understand processes ranging from the effect of the phase transitions at depths of 410 km and 660 km on the patterns of flow in the mantle to the effects of mineral reactions occurring along the pressure-temperature paths followed by crustal rocks. It is therefore necessary to know not only the properties of minerals, but the changes in these properties accompanying such phase transformations and transitions. The following contributions illustrate the need to apply a broad range of experimental techniques to understand phase transitions and transformations. Methods employed range from spectroscopic and X-ray diffraction studies carried out on minerals held in-situ at high-pressures to the careful textural and microstructural examination of samples recovered from both natural systems and laboratory experiments to reveal the phase transitions undergone by minerals at the pressures and temperatures of the Earth's interior. Further insights are gained by the examination of the behaviour of analogue systems of two types. First, chemically pure synthetic endmember systems are studied to determine the principles governing the transformation behaviour of real minerals because in the former the behaviour is not moderated by defects. For example, the studies of phase transitions in MgSiO₃ garnet provide important insights into the possible behaviour of enstatite-pyrope garnets in the Earth's transition zone. Second, the study of non-mineralogical compounds as structural analogues allows us to test our ideas about the factors that govern transformation behaviour in minerals. The example of (Na,K)TiOPO₄, described here, is a prime example as it displays similar behaviour at high pressure to that observed in feldspars.