Abstract

Reviewed are the characteristic mineralogic features that may be used to identify (both on exposure and in alluvial products) rocks bearing ultra-high-pressure (UHP) assemblages, or having undergone UHP. Among the uncommon features of diamond-bearing [crustal?] rocks is the presence of sodium in garnets coexisting with Na-rich clinopyroxene, and of potassium in clinopyroxene. Attention is drawn to zircon as a very safe container of UHP relics, and to microdiamonds and carbonados (and their isotopic properties) as tracers of “diamond-grade” metamorphism. Diamonds themselves, especially microdiamonds from metamorphic rocks, are different from kimberlitic and lamproitic diamonds in both their dominating cubic morphology and their anomalous isotopic composition of carbon depleted in 13C. Caution is urged and some advice given for the identification in thin section of microdiamond and of quartz pseudomorphs after coesite. Uncommon or new minerals found as primary phases in coesite-bearing crustal rocks are bearthite, Ca2Al-(PO4)2OH, an accessory that is also stable at low pressures; ellenbergerite, (Mg, Ti,Zr,□)2Mg6Al6Si2Si6O28(OH)10, a rock-forming mineral showing complete solid solution with an isostructural new Mg-phosphate and so offering the first natural example of a complete silicate-phosphate series (the silicate is stable above 27 kbar and below 750°C, and the phosphate above 10 kbar, so providing geobarometric prospect for the Si-P substitution).