9. System Enstatite–Diopside–Jadeite

Pyroxenes rank high among the most important rock-forming minerals and, because of their optimal chemical and structural complexity, are the most useful minerals for geothermobarometry. Natural pyroxenes stable at the relatively low pressures of the Earth’s crust and shallow upper mantle can contain a large number of components [387]. With increasing pressure, the solubility of some components decreases, which leads to the simplification of the pyroxene chemistry. At pressures above 10 GPa, the content of the tetrahedral Al becomes negligible, which eliminates the Tschermak components, (Ca,Mg,Fe)Al₂SiO₆. In the stability field of stishovite at pressures higher than 9 GPa, the Eskola components, (Ca,Mg,Fe)_0.5AlSi₂O₆, also become negligible [242], because the partial molar volume of SiO₂ in the pyroxene structures is larger than the unit-cell volume of stishovite [10]. The Fe content of the mantle pyroxenes is expected to be low, because the bulk Fe/(Fe + Mg) ratio of an average mantle is only about 0.1, and Fe is partitioned least into pyroxenes. The NaPx component could be present in the mantle pyroxenes at pressures higher than 10 GPa [15], but may require unusual bulk compositions with an excess of Na over Al. That leaves only three major components necessary to consider in the composition of the pyroxenes in the deep mantle: enstatite, diopside, and jadeite. With the further increase in pressure, these pyroxenes transform into majoritic garnet, which becomes the dominant phase in the transition zone. Hence, the phase relations in the ternary system enstatite-diopside-jadeite are particularly important for estimating the mineralogy, chemistry, and structure of the deep mantle.