QEMSCAN® clay mineral identification

QEMSCAN®  compositional maps of selected Source and Special Clays and other reference material providing examples for the kaolin, smectite, illite and chlorite mineral groups.

Clay minerals are the product of chemical weathering, diagenesis and hydrothermal alteration of rocks. They are ubiquitous on earth and comprise a wide range of very fine-grained, layered, and often plastic aluminium silicates. The primary residual alteration products are easily eroded and moved by wind and water. As a result, extensive sedimentary accumulations of clays form in low-energy depositional environments such as lake beds and on the ocean floor. These deposits undergo diagenesis and the resulting materials are referred to as mudstone and shale.

Clays are among the most important minerals used in numerous applications by manufacturing and environmental industries. Some of their unique physical and chemical properties include the high surface-area-to-volume ratio, and high cation-exchange and swelling capacities. These properties are expressed in the characteristically high plasticity and adsorption qualities of some clay minerals.

Clay minerals have important applications and implications in the natural-resource industries, particularly in petroleum exploration and production, and in mining and mineral processing.

Clay minerals occur in all rock formations of siliciclastic petroleum systems, including source, reservoir and seal rocks. While playing a fundamental role in acting as impermeable barriers "trapping" the buoyant hydrocarbons in subsurface reservoirs, clay minerals can also pose significant challenges to exploration efforts and reservoir management.

The presence of clays in ore is a significant mining challenge. Ore bodies are typically marked by a close spatial relationship between fresh and weathered clay-rich zones, with different processing requirements. Small particle sizes and large surface areas result in high chemical reactivity that makes clays very responsive to changes in the mineral processing environment. As a result, mining, throughput, and recovery rates, can be significantly impacted by clays and require changes in the design of the process circuits.

At FEI Natural Resources, we have developed a clay mineral identification protocol using the new QEMSCAN® Spectral Analysis Engine at 20keV to discriminate important clay minerals in natural-resource applications. The protocol has been successfully applied to reference material from the Clay Minerals Society including the Source and Special Clays shown in the figure above. The work has been presented at EUROCLAY in an oral presentation earlier this year.

Environmental mineralogy by QEMSCAN®

Resin-impregnated plug recovered from a core taken from the Hayle Estuary, Cornwall, UK, showing the impact of historical mining. The image shows pre-mining sediments in purple (mostly marine-derived carbonate sands) overlain by laminated muds (brown/red-purple) containing heavy minerals such as cassiterite, pyrite, chalcopyrite, sphalerite and galena.
The image is 27 mm diameter scanned at 10 microns X-ray resolution.

The importance of understanding mineralogy related to contaminated soils and sediments has been highlighted for a number of years by researchers  at the Camborne School of Mines, University of Exeter, UK. Duncan Pirrie, Gavyn Rollinson and Matthew Power have examined samples taken from both estuaries and contaminated land (e.g. Pirrie et al. 2009*).

During these studies automated mineralogy (QEMSCAN®) has successfully been used as a tool to help characterise mineralogy, locate trace phases and determine diagenetic alteration that may lead to bio-availability of heavy metals. In combination with bulk chemistry and mineralogy techniques such as x-ray fluorescence (XRF) and X-ray diffraction (XRD), our understanding of environmental mineralogy can be improved and environmental processes be better understood and managed.

* Pirrie, D., Rollinson, G.K., Power, M.R. 2009. Role of automated mineralogy in the assessment of contaminated land. Geoscience in SW England, 12. 162-170.

Mapping the world ... one micron at a time

Watch out for the August issue of Elements, the International Magazine of Mineralogy, Geochemistry, and Petrology. On page 228 you will find the first in a series of thematic advertisements of FEI's SEM-based Petrographic Analyzers. Elements publishes peer-reviewed papers which are part of a theme based collection. This issue is on "When the Continental Crust Melts", and FEI Natural Resources contributed with an amazing QEMSCAN® image of a garnet schist from Brittany, France.

The compositional map of the schist shows spectacular regional metamorphic textures. Schistocity is the term referring to a mode of foliation typical for medium-grade metamorphic rocks, where platy minerals such as micas and clays (in shades of green) are parallel aligned. Quartz (pink) occurs in form of elongated, drawn-out grains. The garnet group crystals (purple) are hexagonal in cross-section and really stand out in this piece of art. Garnets are important minerals in establishing the temperature-time histories of metamorphic rocks.

We would like to take this opportunity and thank Michael Garrick for providing the sample, a beach pebble he picked up as an undergraduate geology student. We used this sample to develop QEMSCAN® mineral identification protocols based on the latest generation of the Spectral Analysis Engine in iDiscover version 5.x.