How to identify pigeonite: A Raman and SEM-EDS study of detrital Ca-poor clinopyroxene from continental flood basalts

Pigeonite is a high-temperature calcium-poor clinopyroxene commonly found in subalkaline mafic lavas but generally overlooked in provenance studies because of its difficult identification. This methodological study, using as examples detrital pyroxene grains derived from the Karoo and Paraná-Etendeka continental flood basalts exposed in southern Africa and South America, indicates how pigeonite can be reliably discriminated from other pyroxene species during routine provenance analysis based on optical and Raman methods. Under the microscope, pigeonite is distinguished from augite and diopside by its much smaller 2 V angle. Under the Raman spectroscope, pigeonite is readily distinguished from augite and diopside, which show one peak at ∼666 cm−1 instead of the ν3 (∼ 655 cm−1) - ν4 (∼672 cm−1) doublet in pigeonite, and from orthopyroxene, showing a peak at ∼75 cm−1 absent in pigeonite. Otherwise, both pigeonite and orthopyroxene display two doublets, ν3 - ν4 at ∼660 cm−1 and ν5-ν6 at ∼1000 cm−1. Safe identification of pigeonite thus requires a precise measurement of peaks position that, combined with several plots proposed in this study, also allow an indirect estimate its magnesium content. During slow cooling, pigeonite can invert to an intergrowth of augite and orthopyroxene named “inverted pigeonite”. Linear and 2D Raman maps of inverted pigeonite are here presented to highlight the mottled structures of pyroxene grains composed of coexisting pyroxene phases.