X-Ray Diffraction (XRD)
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 recognized 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 3% 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.
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.