9 - X-Ray Diffraction
Most minerals are crystalline and therefore scatter X-rays in a regular, characteristic way dependant on their crystal structure. Each mineral produces a unique diffraction pattern and can be identified from that pattern like a fingerprint.
Identification of minerals is made by comparing their diffraction patterns with a library of over 17,000 mineral patterns stored in the International Centre for Diffraction Data (ICDD). Detection limits depend on the sample. For geological material, it is estimated that the minerals present in less than 2% of the sample might not be detected.
The samples for X-ray diffraction analysis are ground or milled to a fine powder and then hand pressed into the sample holder. Approximately 1cm3 of the material is sufficient for rock mineral analysis but smaller amounts can also be accommodated by using a low background holder.
Mineral Identification is available in three levels of accuracy:
- minerals are identified and their amounts determined using the Rietveld method. Corundum is added to the sample as an internal standard in order to determine the amount of X-ray amorphous material
- minerals are identified and their amounts determined using the Rietveld method
- minerals are identified however their amounts are not determined
X-ray diffraction, combined with dedicated sample preparation, is the most effective technique for precise identification of clay minerals. The amount of sample required for clay speciation depends on the amount of clay fraction present. Since several oriented mounts have to be prepared, the required amount of sample is typically 10 – 15 g or more. Samples must be submitted unprepared.
The quantification of crystalline silica phases (Alpha Quartz) in bulk samples is carried out as an extension of the usual quantitative XRD procedures (absorption correction, internal/external standard methods and whole pattern analysis) depending on the sample. Several grams of sample are required.
In cases where industrial applications may cause silica to become airborne, the samples are collected and analyzed on filter membranes, following the guidelines of NIOSH 7500 or OSHA ID-142 Methods for respirable Alpha Quartz.
Determination of concentration of amorphous silica is performed following NIOSH Method 7501.The method uses the property that most amorphous forms of silica transform to cristobalite with heat treatment at 1100 °C or 1500 °C. After firing, the sample is analyzed for cristobalite following steps similar to Method 7500.
At Actlabs, we determine the XRD spectrum and use cluster analysis, a statistical tool that groups XRD patterns into clusters based on the similarity of their peak and profile information. It can be used to rapidly group large data sets into smaller clusters with more similar mineralogy. Selected samples in the cluster can have Rietveld quantitative determinations made. This has potential applications for mining and exploration, wherein clustering would highlight existing changes in mineralogy within a deposit. Clustering would also be useful for determining mineralogical variability, which may have metallurgical implications.