Interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data

TLDR

Clay mineral crystallinity and crystallite size data from X‑ray diffraction are widely used to characterize very low‑grade metamorphism, but interlaboratory differences in equipment, methods, and sample preparation make comparisons unreliable. The authors propose a calibration approach that uses rock‑chip standards to standardize these data, enabling direct quantitative comparison across research groups. They introduce a crystallinity index standard (CIS) with illite anchor limits of 0.25 dΔ2θ and 0.42 dΔ2θ, corresponding to crystallite sizes of 52 nm and 23 nm, determined by the Warren‑Averbach method. Applying this calibration to both legacy and new data improves the reliability of comparisons and correlations with other indicators of very low‑grade metamorphism, thereby enhancing the value of clay‑mineral studies.

Abstract

ABSTRACT Clay mineral crystallinity and crystallite (domain) size data determined by X‐ray diffraction (XRD) are methods extensively used in the characterization of very low‐grade metamorphic conditions. However, the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained, a consequence of the different machine conditions, measurement methods and sample preparations used during analysis. A calibration approach to the standardization of data using rock chip standards is presented, which allows data sets produced by different research groups to be directly and quantitatively compared. A standardized scale, the crystallinity index standard (CIS), is proposed, with illite crystallinity anchizonal boundary limits of 0.25d̀Δ2θ and 0.42d̀Δ2θ, and equivalent illite crystallite sizes of 52 and 23 nm, respectively, determined by the Warren‐Averbach method. Calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments, and should improve the accuracy and reliability of correlations made between XRD data and other indicators of very low‐grade metamorphism, hence increasing the value of such clay mineral studies.