Alistair J.R. White

Vibrational spectroscopy of epidote, pumpellyite and prehnite applied to low-grade regional metabasites

Calc-silicate minerals are routinely used to identify metamorphic and metasomatic patterns, but their characterization usually requires detailed and time-consuming petrographic and geochemical analyses, preventing spatially extensive mapping. Laboratory measurements (FTIR, XRD, EPMA) successfully characterize pure representatives of three calc-silicate mineral groups – epidote, pumpellyite, and prehnite – as well as mixed mineral samples from the Fortescue Group volcanics, Hamersley Basin, Western Australia. Pure mineral spectra display diagnostic first overtone bands of the hydroxyl-related stretching fundamentals in the 1300 – 1600 nm wavelength region. In mixed mineral assemblages the overtone region allows for identification based on discrete absorption bands: 1555 nm for epidote, 1470 nm for prehnite, and four bands at 1410 (a), 1430 (b), 1450 (c) and 1510 nm (d) for pumpellyite. With increasing Fe content, the wavelength positions of the hydroxyl-stretching fundamental and first overtone absorption features of epidote and prehnite shift towards shorter wavelengths. In pumpellyite, with increasing Mg content, bands (b) and (d) shift towards longer and shorter wavelengths respectively. Diagnostic overtones are identifiable after downsampling of laboratory FTIR spectra to hyperspectral field, airborne, and orbital spectrometer resolution. This highlights the application to regional-scale mapping of calc-silicate distribution and composition, both on Earth and on extraterrestrial planetary surfaces, such as Mars.