Automated mineralogical methodology to study carbonate grain microstructure: an example from oncoids

Oncoids collected from an arid intertidal embayment of the Gulf of Aqaba, Saudi Arabia, were examined for their morphology and mineralogical composition. This study tested a methodological approach for studying oncoid microstructure and chemical/mineralogical composition in detail. The method was applied to more than 30 oncoid samples, and examined internal morphology and composition via automated mineralogy and petrographic methods, deploying quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN). The QEMSCAN analysis system gathers X-ray and backscattered electron data from predefined points on a sample and interprets it in mineralogical terms by comparisons with a spectral library. Volumetric modal analysis, a function of the cumulative data obtained, is then used to quantify the mineral species present in the sample. The most abundant minerals in this study consist of calcium carbonate (60-70 %). Quartz dominates the detrital trapped grains (19.5 %). Potassium feldspar and albite constitute 5.6 and 3.4 %, respectively. Minerals such as illite, chlorite, biotite, muscovite, smectite, kaolinite, apatite, and ilmenite and siderite individually make up less than 1 % of the overall oncoid composition. Unlike traditional light microscope petrography, X-ray fluorescence and X-ray diffraction methods, the proposed technique has several advantages including, but not limited to, preservation of the oncoid microstructure, porosity and pore size estimation, elemental distribution of major minerals, determination of the chemical composition of grains as well as the cement, and relatively high accuracy of elemental compositional data taken from polished sections. This study shows that QEMSCAN can provide an effective and robust tool in analyzing different components of oncoids and other carbonate grains.