CLASSIFYING CRYSTALLINITY AND MINERALOGY IN DEVITRIFIED RHYOLITES THROUGH THE USE OF X-RAY DIFFRACTION

Devitrification, the process by which glassy substances become crystalline, has a large impact on lava flow emplacement and the stability of lava domes. In order to understand the processes controlling devitrification and the timing of devitrification relative to cooling, we must first investigate how best to characterize devitrification products. In the present study we investigate changes in the mineral assemblage and proportions, and crystallinity of various rock samples.

Powder X-ray diffraction techniques to classify crystallinity of materials are commonly utilized by polymer/material sciences, however these techniques have not previously been utilized for geological applications. The crystallinity of the sample is calculated by taking the integrated area of the X-ray diffraction pattern and separating it into crystalline and amorphous (glassy in the case of volcanic rocks) components. By doing this we can estimate the percent crystallinity of the sample. This technique may be applied to glassy rocks to readily determine the bulk crystallinity of the sample, or to groundmass separates to investigate the degree to which the groundmass of the sample is crystalline. To validate the technique we combined various proportions of entirely crystalline (granite) and entirely amorphous (a glass plate) materials in known weight proportions to construct a calibration curve. The agreement between the known and calculated crystallinity of the calibration curve is excellent indicating that this is a robust technique.

Crystallinity results for high-grade, welded ignimbrite samples as determined via X-ray diffraction indicate varying crystallinity (~10-100%) for both groundmass and bulk rock. In addition, the proportions of cristobalite and tridymite systematically vary through the ignimbrite suggesting a possible correlation to cooling rate. Powder X-ray diffraction techniques therefore appear to provide a robust method for determining the crystallinity and mineral proportions of devitrified volcanic materials- a vital step in being able to quantify how physical properties of the volcanic dome or flow may change with cooling.