Abstract

A multi-methodological approach that consists of the systematic integration of detailed mineralogical, micromorphological, and geochemical features has been used to characterize a number of heterogeneous siliceous Paleozoic materials including low- to very-low-grade metamorphic crystalline rocks and siliciclastic and other detrital materials Ordovician to Carboniferous in age. Included in the same sequence are black slates, quartzites, and shales with alternating beds of sandstone, mudstone, and, occasionally, intermediate coal beds. A set of complementary and sequential analyses was performed to confirm the primary mineral assemblages, particularly the presence of unusual secondary silicates in the clay fraction. The sequential mineralogical methods included microscopic petrographic analysis (MOP), scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDS), and X-ray powder diffraction (XRPD). Major elements were analyzed by X-ray fluorescence spectrometry (XRF) and minor elements (in some cases) using inductively coupled plasma mass spectroscopy (ICP-MS). Mineral quantification was carried out using the Siroquant system which is based on the QXRPD Rietveld technique. The essential minerals are mainly represented by K-rich dioctahedral varieties of white mica (muscovite) or illite, Fe-rich chlorites (Mg-rich chamosites), plagioclases (albite), and quartz, occasionally with kaolinite or even gibbsite. Up to 12 trace minerals such as iron oxides and sulfides were identified in some assemblages. After performing a validation procedure, a high level of consistency was observed between the major element oxide percentages, inferred from the XRPD study, and the equivalent oxide proportions determined by direct chemical analysis. The strong linear correlation shows that the percentages of K2O, Na2O, and MgO are also consistent with the quantitative clay mineralogy. The abundance of albite and the presence of iron-rich chlorites, together with the identification of kaolinite and occasionally gibbsite, support the interpretation of these bedrock materials.

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