QEMSCAN: A REVOLUTIONARY SEDIMENTARY PETROLOGY TOOL

QEMSCAN (Quantitative Elemental Mineralogy using a SCANning electron microscope) is a Zeiss scanning electron microscope with four Bruker high-speed energy dispersive detectors that can produce a detailed mineralogical ‘image’ of a polished thin section or epoxy plug. QEMSCAN has become one of the essential laboratory tools in the mining industry for ore characterization, yet QEMSCAN has important applications for sedimentary studies as well.

QEMSCAN technology identifies variations in mineral grains and cement phases present in minor amounts (1 µm² areas). This tool rapidly acquires accurate and abundant digital point count data, including porosity. Minor variations in cement mineralogy, mineral chemistry and textures allow documentation of chemical and mineralogical gradients and mineral paragenesis.

We examined Jurassic Navajo Sandstone spheroidal hydrous ferric oxide (HFO) concretions because they provide a valuable record of the complex diagenetic history involving multiple precipitation events. These concretions commonly consist of a well-cemented rind surrounding an interior relatively depleted of HFO cement. QEMSCAN data differentiate textural phases of HFO, distinguish rind cements from interior cements, and determine porosity for each area. HFO nucleates occur on some feldspar grains. This suggests that feldspar hydrolysis raises the pH locally and nucleates HFO to form concretions. Analysis with iDiscover software shows that the MnO occurs as a late-stage pore filling, and indicates a rising pH in the precipitating fluid.

QEMSCAN accurately performs point counts of entire samples (or parts of samples) in a fraction of the time required using traditional methods. iDiscover software has the ability to digitally isolate minerals to determine composition or to detect the presence of mineral inclusions. Pore and grain geometries are easily classified with iDiscover and the software is used as a “virtual sieve” for grain size classification. This tool provides data that are difficult to obtain via other traditional methods (such as XRD) or would entail multiple analyses to acquire (such as microprobe, SEM, and optical microscopy). QEMSCAN is an important component in the future of sedimentary petrology.