Abstract
This study presents a cross-correlated surface and near surface investigation of two phlogopite polytypes from Kasenyi kamafugitic rocks (SW Uganda) by means of advanced Atomic Force Microscopy (AFM), confocal microscopy and Raman micro-spectroscopy. AFM revealed comparable nanomorphology and electrostatic surface potential for the two mica polytypes. A widespread presence of nano-protrusions located on the mica flake surface was also observed, with an aspect ratio (maximum height/maximum width) from 0.01 to 0.09. Confocal microscopy showed these features to range from few nm to several μm in dimension, and shapes from perfectly circular to ellipsoidic and strongly elongated. Raman spectra collected across the bubbles showed an intense and convolute absorption in the range 3000–2800 cm−1, associated with weaker bands at 1655, 1438 and 1297 cm−1, indicating the presence of fluid inclusions consisting of aliphatic hydrocarbons, alkanes and cycloalkanes, with minor amounts of oxygenated compounds, such as carboxylic acids. High-resolution Raman images provided evidence that these hydrocarbons are confined within the bubbles. This work represents the first direct evidence that phlogopite, a common rock-forming mineral, may be a possible reservoir for hydrocarbons.
Highlights
Hydrocarbon fluid inclusions (HCFI) occur in fractures and microcracks of rocks and veins within forming minerals, e.g. quartz, calcite and pyroxene, and may consist of hydrocarbon oil, H2O, gases such as CO2 and CH4 or solids entrapped along with/or precipitated from the liquid phase at later times[1,2]
The Atomic Force Microscopy (AFM) was used to investigate the in situ K+ − Na+ ion exchange in phlogopite single-crystals[10]
The selected phlogopites were prepared for the AFM observations by fixing them onto AFM specimen mounting metal discs with double-sided adhesive tape
Summary
Hydrocarbon fluid inclusions (HCFI) occur in fractures and microcracks of rocks and veins within forming minerals, e.g. quartz, calcite and pyroxene, and may consist of hydrocarbon oil, H2O, gases such as CO2 and CH4 or solids entrapped along with/or precipitated from the liquid phase at later times[1,2]. Hydrocarbons may derive by biogenic (thermogenesis; bacteriogenesis) or abiogenic processes. The latter are generally based on the reduction of carbon dioxide during magma cooling or, in hydrothermal systems, during water-rock interactions leading, for example, to serpentinization of ultramafic rocks. The analysis of the HCFI may be performed on the bulk rock by crushing of the sample or on single inclusions by means of micro-spectroscopy (e.g. optical, infrared, Raman and fluorescence spectroscopy). Observation of widespread nano- to micron-sized bubbles on the mica flakes prompted this multidisciplinary study, carried out by combining AFM, confocal microscopy, and Raman imaging
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