Saturday, 30 July 2011

QEMSCAN studies of metamorphic rock samples

Identification of key minerals is of great importance to determining the tectonic history of metamorphic samples. These key minerals may be few in number and present only as small micro-inclusions making them difficult to identify, if at all, with a petrographic microscope.

Once relict minerals of earlier metamorphic assemblages are located, thermobarometry and geochronology can then be applied, resulting in a wealth of information on previous segments of the pressure-temperature-time-deformation path. The relict mineral textures and their relationship to the fabric of the entire thin section can be easily seen in mineral maps yielding important textural information.

The QEMSCAN at the Camborne School of Mines (CSM) is being used to study metamorphic samples from the Central Metamorphic terrane of the Eastern Klamath Mountains, Northern California.

Intial results were presented at the European Geosciences Union, General Assembly, Vienna, 2-7 May 2010 "Application of Automated SEM-EDS Based Mineral Identification Systems to Problems in Metamorphic Petrology" by Robert Fairhurst, Wendy Barrow and Gavyn Rollinson.

Friday, 29 July 2011

QEMSCAN Filament Life

Currently the tungsten filament in our QEMSCAN system Camborne School of Mines, University of Exeter, UK has been running for 6376 hours (since 15th December 2011) which is well over 8 months. Is this a record for tungsten filaments? Comments welcome on life spans of your filaments!

QEMSCAN: Beyond Minerals

Automated Mineralogy can characterise more than just minerals. At the Camborne School of Mines (CSM), University of Exeter, UK, we have been experimenting with a wide range of sample types for over 7 years. To the left is an example of a historical smelter slag (from historic mining activity pre-1900's) taken from Calenick Creek, near Truro, Cornwall, UK.

The image shows mainly Fe-As (blue) and Sn metal (red) phases, and identified various other phases. Sample size is approx. 4 x 2 cm, fieldscan mode at 10 microns resolution with about 2 million analysis points.

TOP 10 AM papers: Riley et al. 1989, Hydrobiologia 176/177, 509-524

QEMSCAN mineral map of fluvial sediments collected from the 2010 Brisbane flood
Review of Riley et al. 1989, Hydrobiologia 176/177, 509-524

In terms of early adopters of automated mineralogy technology beyond mineral processing, due credit must be given to Riley, Creelman, Warner, Greenwood-Smith and Jackson from Australia. In their groundbreaking journal paper on "The potential in fluvial geomorphology of a new mineral identification technology (QEM*SEM)" they pioneer the application of automated SEM-EDS compositional mapping to the study of depositional environments and sediment sources. I only became aware of this paper after publishing a similar study on late Pleistocene flood deposits in the Flinders Ranges, South Australia as part of my PhD thesis, and would like to take this opportunity to give due credit to a study well ahead of it's time.

Riley et al. set out explaining why mineral analysis remains an underutilized diagnostic tool in fluvial geomorphology and sedimentology. Optical microscopy on fine fluvial sediments is cumbersome, statistically questionable and of limited value when it comes to discriminating source areas based on clay mineralogy.

The objective the authors face is to establish the maximum flood level the Nepean/Warragamba Rivers in New South Wales over the recent geological past. The deeply entrenched river terraces in the lower reaches are inappropriate indicators because they represent a different hydrological regime. In the absence of slack water deposits, veneers of alluvium mixed with colluvium by bioturbation remain the only record.

The task was to perform micron-scale textural analysis and discriminate non-fluvial from fluvial material as well as to quantify the sediment contributions from different tributaries in the catchment. Challenges in fingerprinting and provenancing fluvial sediments by mineralogical analysis include sorting and differential comminution during transport, post-depositional weathering, contamination by reworking and aeolian deposition, and mixing of sediments from different source rocks. All this requires a focus on minerals resistant to weathering, and sophisticated statistical analysis of the data.

The methodology section is exceptional. The paper provides detailed information on the QEM*SEM system configuration including a schematic diagram and a technical discussion of the data acquisition and processing in the appendix. The authors developed their own application-specific mineral identification protocol (SIP) and primary mineral lists, differentiating readily identifiable mineral species from broadly related silicate groups. This was at a time when no interactive Measurement Debug module for SIP development was available as in later versions of the iDiscover software package. Unfortunately, it was also before the powerful categorizer tools were developed that provide classification of particles by mineral association, size and shape. As a result, the authors had to make do with modal mineralogy data for 4 physical size fractions. Riley et al. clearly set an example by applying principle component analysis to produce and assess independent variables. The spatial relationship between minerals from different sampling locations was investigated by multiple cluster analysis.

The results are convincing and clearly differentiating fluvial from non-fluvial deposits by QEM*SEM mineralogical data. The conclusion was that the estimate of Probable Maximum Flood discharge for the downstream dam had to be revised. The conclusion after reading this paper is that sedimentologist can clearly benefit from revisiting this pioneering paper before applying the latest image analysis capabilities that compositional mapping solutions such as QEMSCAN provide.

Wednesday, 27 July 2011

TOP 10 AM papers: Grant et al. 1976, Scanning Electron Microscopy/1976 (III)

1982 prototype of QEM*SEM with mini-computer to the right at CSIRO Melbourne

Review of Grant et al. 1976, Scanning Electron Microscopy/1976 (III)

In the 70's, it was not uncommon to publish outstanding geoscientific research in workshop proceedings. This groundbreaking paper on "Multicompositional particle characterization using the SEM-microprobe" by Grant, Hall, Alan Reid and Martin Zuiderwyk is published in the Proceedings of the Workshop on Techniques for Particulate Matter Studies in SEM held at the IIT Research Institute in 1976. Over three decades ago, the authors - listed in alphabetical order - demonstrated the first computer-controlled automated mineralogy system and outlined a number of principle functions in mapping particles which changed little over the years despite the revolution in computational power and software languages since.

Grant et al.'s vision, as outlined in the introduction, is to determine sizes and composition of complex particles, and to "measure rather than infer" areas and perimeters to derive shape functions to better understand physical and chemical behaviors of particles in industrial and mineral treatment processes, i.e. mineral flotation and the degree of liberation during grinding processes.

The original instrument design consists of a mini-computer controlled e-beam which is automatically moved across the sample along a user-defined pattern. An initial "fast scan" locates the particles for detailed scanning. The dwell time for x-ray acquisition at each point can be defined. In addition, an "event acceptance filter" is in place to only record changes in material composition. Composition and e-beam coordinates are saved in form of digital maps composed of line segments and points.

The original system is setup to accept secondary and backscattered electron signals, absorbed specimen current, as well as energy-dispersive x-ray counts from either EDS detectors or microprobe. The measurement mode outlined is a proto-type for what is to become the Particle Mineral Analysis (PMA) in the QEM*SEM and later QEMSCAN solution. It uses either rapid BSE or SE signals to locate the edges of particles which are in turned scanned in detail using x-ray signals. The software includes algorithms linking particles extending across multiple frames, similar to the "field stitch" pre-processor in iDiscover. In addition, mixed signals between particle and mounting medium, referred to as "boundaries" are resolved, a first step in the development of Species Identification Program (SIP) boundary phase definitions and the award-winning "Boundary Phase" pre-processor by Paul Gottlieb in the iDiscover software package. Touching particles are discussed beyond shape parameters using secondary electron images to discriminate same phases in the discussion with reviewers at the end of the paper.

The boundary coordinates are saved allowing for a visual display of the data and basic image analysis functions, including the particle-by-particle calculation of area, perimeter, centroid and even the option to report phase contributions to the perimeter. Particular consideration is given the stereological challenges of reporting unbiased particle size, shape and composition from 2-D data. Interestingly, the authors point out to future investigations in mounting particles onto surfaces and using a second set of detectors providing biaxial views to better estimate particle sizes in three dimensions.

It can be fairly said that the authors laid the foundation for automated mineralogy and future software developments. It is a testimony to their visional capacity that they discus ways to move forward in 3-D particle analysis and even applications of their algorithms beyond rock particles, such as the analysis of pore space or images of macro-scale objects.

Thursday, 14 July 2011

Automated Mineralogy publication trends

Peer-reviewed journal and conference papers on automated mineralogy applications as of mid-2011

There are now >100 papers on automated mineralogy which I have compiled using the open source reference management software Zotero. The library includes book chapters, journal papers, peer-reviewed conference papers, public reports, research thesis, as well as important press releases on automated SEM-based solutions for compositional mapping of minerals and rocks. The bibliographic information is openly accessible via the Internet as a group library with closed membership.

In the process of compiling the literature and organizing the papers by publication type, field of application, as well as analytical technology solution, some interesting trends emerged. The most significant one is a marked increase in automated mineralogy publications over the past couple of years. This year alone, papers published in 2011 or in press account for nearly one third of all automated mineralogy publications.
The two industry-standard automated SEM-EDS solutions QEMSCAN® (formerly QEM*SEM) and the Mineral Liberation Analyzer (MLA) make up the large majority (>90%) of the published literature. This literature is dominated by papers applying QEMSCAN® in areas ranging from oil and gas, coal and flyash, ore characterization, mineral processing optimisation, environmental mineralogy, archaeology and forensics. Papers based on the MLA solution are more focused on the optimization of mineral processing and bright phase mineral detection.

Considering the dominant position of QEMSCAN® in the scientific literature (~80%), it is an interesting exercise to compare the historical stages of QEMSCAN® development with the scientific output. As QEM*SEM (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) was first developed at CSIRO Australia, a limited number of authors lead by the developers such as Alan Reid, Paul Gottlieb, David Sutherland and Alan Butcher laid out the methodological groundwork for early applications. In 2003, when QEM*SEM was turned into a commercial solution by Intellection, publication output picked up by early adopters such as Norman Lotter and Duncan Pirrie, but the overall volume remained low. Scientific writing gathers momentum in 2006, coinciding with the first international meeting to focus on automated mineralogy technologies held in Brisbane: the Automated Mineralogy ’06 conference which was co-organized by MEI and Intellection. The most significant increase in scientific productivity aligns with the acquisition of QEMSCAN and MLA technologies by FEI, a leading developer and provider of scanning electron microscope and focused ion beam solutions.