Discovering High-Pressure and High-Temperature Minerals

Abstract

Defining high-pressure (P) and high-temperature (T) minerals beyond vague conventions requires robust criteria. The conjunction of mineralogy and (mantle-)geochemistry suggests that pressure-dependent ionic radii provide such a criterion. A set of quantitative arguments is provided based on the pressure-dependent radii of several elements. Three categories and regimes of high-P minerals are defined. All approved high-pressure minerals are tabulated here. High-pressure minerals form under static and dynamic pressure. Under dynamic compression the short duration of the peak pressure state acts as a kinetic barrier for transformations. Only local high temperature (‘hotspots’) permits formation of high-pressure minerals. Very high temperature of extreme shock compression induces retrograde conversion of high-pressure minerals or melting during the passing of the rarefaction wave. Only few metastable high-pressure silicate minerals (and even synthetic phases) have been observed in shocked rocks and samples: Even along temperature gradients we find metastable formation of phases stable at lower static pressures but few minerals without stability field, despite the multitude of possible metastable structures. This suggests sterical hindrance of the Si[4] → [6] transition, besides the kinetic barrier. In the deep Earth high-pressure minerals in the deep Earth are hidden from direct observation. Hypothesized retrograde transformations in peridotites and of inclusions in diamonds remain to be confirmed. Few occurrences of high-pressure minerals as inclusions in diamonds have been reported. In conjunction with their hosting mineral, diamond, they appear to have formed in regions of mantle metasomatosis, and potentially mark regions or horizons of extensive chemical mobility within the mantle. Consistent with the definition of high-P minerals we define a high P–T regime and we propose to define high-T minerals that form at low or ambient pressure through the T-induced changes in redox buffer systems. This approach encompasses the rich mineralogy of presolar and early solar minerals which cover a compositional range far beyond the occurrences in differentiated planetary bodies like Earth, Mars, and Moon.