Today I had a question about the need to use X-Ray Diffraction (XRD) for bulk mineralogy on a head sample destined for detailed QEMSCAN mineralogical analysis. This is an excellent question as it encompasses some fundamental aspects of how QEMSCAN works and the relative strengths of e-beam systems for mineralogy as opposed to more traditional methods.
For those readers familiar with setup of QEMSCAN test programs a standard test is preliminary analysis of the bulk sample by XRD. In combination with elemental analysis this is a vital step, especially for unknown samples. It develops a level of understanding about the material that can focus the QEMSCAN analysis and help give sufficient information for important processes such as SIP development.
It may appear that the information generated by XRD is made redundant by more detailed information generated by QEMSCAN further down the project workflow. It is true that an accurate mineralogical balance can easily be generated from QEMSCAN mineralogy on sized fractions if the mass distribution is known. This will give an idea of the bulk mineralogy but will be subject to any errors that may have been introduced by statistical factors or errors in the mass distribution. The bulk mineralogy generated by XRD will provide a validation tool for the accuracy of QEMSCAN mineralogy by back-calculation.
Simple validation of data is only one aspect that XRD brings to QEMSCAN programs. Of greater importance is the accurate definition of major minerals that can be used in Species Identification Protocol (SIP) development. For those unfamiliar with how QEMSCAN identifies minerals, the SIP is a master list and rules that assigns X-Ray spectra generated for each pixel to a mineral group. The rules work on a hierarchy and can allow for broad or narrow elemental distributions within the specta depending on whether mineral groups or specific minerals require definition. Generic SIP’s are generally available based on key minerals for various commodities; however, these need refinement for each new ore type to capture unique minerals. The use of XRD to quantify the bulk mineralogy of a head sample can greatly assist with this refinement process as key minerals can be targeted and assigned in the SIP.
What this topic also raises is the fundamental differences between XRD and e-beam systems, such as QEMSCAN. Both techniques can define bulk mineralogy and have strengths and weaknesses in this area. Fundamentally, the techniques differ in the manner data is generated. XRD measures the unique diffraction pattern of X-Rays passing mineral crystals. It is generally used as a bulk method with the average diffractogram generated for a pressed powder pellet of material. In contrast, QEMSCAN examines discreet mineral particles in section with an electron beam and determines the mineral composition from elemental composition measured by characteristic X-Rays generated by the material. Where XRD is a bulk technique, QEMSCAN is a microscopic technique.
Generating data in these ways means XRD and QEMSCAN can be used very effectively as complementary techniques. QEMSCAN is very powerful in determining associations of specific minerals through imaging and generating loads of very useful data relating to these areas. QEMSCAN can however be sensitive to statistical errors caused by the relatively small number of particles that can be analysed. XRD cannot give the mapping capabilities of QEMSCAN but removes many of the statistical difficulties. Hence, if used together they provide more robust and useful data.
This is just one area of QEMSCAN program setup that should be understood. I will periodically investigate other areas that I believe are important but if you have any specific questions or areas that you think could benefit from clarification feel free to post a comment and I will look into them.