Recently we encountered a radiating pearly, platy, mineral in metabasalts from Greenvale district , north Queensland. Indicative ID is Pumpellyite , on basis of habit, hardness (although brittle) and lustre, and chemical composition determined from Portable XRF which showed the right proportions of Al, Ca, Si, Mg, minor Fe & Ti. We still have to confirm this with petrology.
Pumpellyite is an index mineral of the Prehnite-Pumpellyite Metamorphic facies , particularly prominent during burial metamorphism of basaltic rocks. Temperature-Depth conditions of formation are in the order of 250oC and 350o C and 6km to 20km (2kilobars to 7 kilobars). From my experience, it is rare to find such coarse crystals of pumpellyite as it is usually only identified from microscopic work. This whole exercise shows the value of using the PXRF for indicative whole rock and mineral ID as such index minerals record the P-T conditions and ,by implication the tectonic history, of the geological terrane they come from.
