FTIR ANALYSIS OF CHERT CRYSTALLINITY: PRELIMINARY STRUCTURAL PARAMETERS FOR FRACTURE ANALYSIS

Fourier Transform Infrared (FTIR) analyses were performed on cherts to obtain crystallinity indices as part of an ongoing fracture mechanics study that began in 2017. Chert crystallinity indices were derived as structural parameters that will be utilized in the ongoing larger project aiming to develop a comprehensive model of chert failure at the microscale. Potassium-bromide pellets of nine different cherts types and a crystalline quartz standard (n=90) were analyzed using FTIR. Infrared spectra in the region 400-4000 cm^-1 were obtained from all pellets at ambient temperature. Pellets were then subject to heat treatment at 100˚c, and their spectra were recorded again. This process was repeated by sequentially increasing heat treatment by 100˚c up to 600˚c. Thus, seven different spectral readings, each recorded after different temperature treatments, were obtained for all pellets. This was done to observe any changes in lattice water and structure. Following after Saikia, ratios were calculated from the intensity of peaks associated with 778 cm^-1 and 695 cm^-1. These ratios represented indices of crystallinity, the degree of structural order, in each chert sample. The relatively low crystallinity indices (< 3.00) observed in this study indicated the cherts were in highly crystalline form. Due to little observed molecular water at ambient temperature, the effects of heat treatment on lattice fluids were negligible. The low crystallinity indices and lack of water suggests that microcrystalline quartz is the principal silica polymorph in the cherts. Degrees of crystallinity will serve as useful parameters in developing integrated models of chert microstructure and failure. Although the cherts in this study were highly crystalline, variation was present amongst their indices. This variation may be representative of past diagenesis. It is known that over time most silica polymorphs in chert eventually crystallize to microquartz. Outside of time, environmental variables and stresses (i.e burial depth, pore pressure, fluid mobility, etc.) also control the mode and extent of silica crystallization in chert. In part chert crystallinity indices have captured past diagenesis and consequently, will allow fracture models currently being developed by the researcher to account for previous diagenetic histories.