On the calibration of the illite Kübler index (illite "crystallinity")

The measurement of the Kübler Index KI [i.e., the full width at half maximum (FWHM) value of the X-ray powder diffractometric first basal reflection of illite - dioctahedral K-white mica, formerly called also illite "crystallinity" (IC)], is rather simple and quick; however, the delimitation of the KI zones still remains controversial at present, mostly because of the numerous factors that affect the standardization and inter-laboratory calibration of the KI scale. The differences reported between the KI values and scales among different laboratories are considered to be due to (1) X-ray diffractometer settings for measurement of FWHM, (2) sample-preparation procedures, and (3) techniques of measuring or obtaining the FWHM values from the line profiles. The effects of (1) are monitored by the use of polished-slate inter-laboratory standards. In order to consider the effects of differences in sample-preparation, Warr and Rice (1993, 1994) distributed inter-laboratory "CIS" standards in the form of (meta)pelitic rock chips and a muscovite flake that require both preparation and measurement in the various laboratories. The "CIS" values given by these authors for these standards were purportedly converted to the Kübler scale using a calibration curve based on measurements on a set of polished-slate slabs and a muscovite flake prepared by Kisch as inter-laboratory KI standards. However, these "CIS" values are much broader than those obtained by virtually all other laboratories, and are considered anomalous; the "raw" data as restored from these "CIS" values are implausibly broad. Comparison with the "raw" values of Warr and Rice suggests that the high- and low-grade boundaries of the anchizone in their "CIS" scale are 0.295 and 0.49-0.50°Δ2θ, much broader than the Kübler-equivalent 0.25 and 0.42°Δ2θ. Similarly, regressions of the "CIS"-equivalent against the slate-slab calibrated Kübler-equivalent values of several laboratories show that the purportedly Kübler-equivalent anchizone-boundary values of 0.25 and 0.42°Δ2θ in the "CIS" scale in fact correspond to much narrower Kübler values. Consequently, the use of the CIS scale boundaries results in increasing confusion when CIS-calibrated KI values are used for petrogenetic purposes in regional comparisons. This discrepancy is likely to reflect errors in the conversion of the "raw" FWHM values into Kübler equivalents. Data on inter-laboratory chip standards are difficult to evaluate unless the "raw", uncalibrated data are also given: such data should be included in all papers reporting on KI.