Quick and Easy Identification of Coesite by Cathodoluminescence Spectroscopy

The presence of coesite in metamorphic rocks is used to demonstrate their ultrahigh-pressure conditions. However, since the simple reaction kinetics of the coesite to quartz backreaction mostly leads to its elimination during exhumation, the presence of coesite is scarce and its finding difficult. Here, we present a new efficient technique for coesite identification, based on its specific cathodoluminescence (CL) emission documented by a scanning electron microscope equipped with a cathodoluminescence spectrometer. The analyzed coesite grains, confirmed by electron backscatter diffraction, come from three distinctly different samples of (i) eclogite, (ii) mylonitic orthogneiss, and (iii) quartz deformation experiment, to cover a wide range of possible chemical and dynamic environments in which coesite forms. The 24 analyzed coesite grains in the three samples show consistent CL spectra characterized by low intensity and CL emission peaks concentrated at wavelengths between 530 and 570 nm. The complementary CL spectra of quartz in the vicinity of coesite grains show one to two orders of magnitude higher intensity and CL emission peaks concentrated at wavelengths of c. 650 nm. Since the CL emission of coesite presented here is distinctly different from the emission in natural quartz reported so far, the coesite spectra, documented in this work, aim to serve as a diagnostic reference material establishing a new technique to identify coesite.