Application of an improved TEM-EDS protocol based on charge balance for accurate chemical analysis of sub-micrometric phyllosilicates in low-grade metamorphic rocks

Determining the chemical composition of sub-micrometer rock-forming minerals is still a challenging task. The electron probe micro-analyzer (EPMA) is considered the most accurate analytical way to obtain chemical data on amorphous and crystalline materials. However, performing EPMA analyses on sub-micrometer-sized grains is uncertain not recommended as the risk of obtaining analyses contaminated from the surrounding phases. A transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectrometer (EDS) provides a greater spatial resolution, making it possible to obtain trustworthy chemical information on sub-micrometer-sized material. In this work, we present a fast and cheap data-reduction protocol for TEM-EDS chemical analysis, where k-factors derived experimentally for each element of interest and absorption correction are implemented. The results are compared with those determined using standardless and non-corrected TEM-EDS protocols. The k-factor for oxygen plays a fundamental role and its value should be calculated from compounds similar to the phase of interest. For absorption correction, the contribution of hydrogen during structural formula recalculation is taken into account, like a lower net valence of oxygen. The robustness of this protocol was tested by performing TEM-EDS analyses on white mica grains from metapelites, belonging to the Internal Ligurian Units exposed in the Northern Apennines, the chemical composition of which is well constrained. Such a protocol has proven to provide high-quality results from both statistical and crystallo-chemical perspectives. Remarkably, the tested data-reduction protocol for TEM-EDS analysis provided chemical compositions consistent with the EPMA results previously obtained from the same samples.