Calibration of the anchizone: a critical comparison of illite ‘crystallinity’scales used for definition

Instrumental settings for determination of illite/muscovite ‘crystallinity’(half‐height width of the 10‐Å X‐ray diffraction peak) and the limits of the anchimetamorphic zone adopted by various authors fall into several groups. The variation in the limiting peak widths between the authors that have adopted Kubler's Neuchâtel boundary values of 0.42° and 0.25°Δ2θ can be interpreted in terms of variation in the instrumental settings. The choice of time constants higher than those given by the formula causes peak broadening; this effect is particularly marked at high scan rates. The peak broadening is by constant increments that are virtually independent of the absolute peak width. The differences between the Kubler (Neuchâtel)‐derived limiting values and some other scales are appreciably greater than can be accounted for by these differences in instrumental settings: many of these scales are not equivalent. In particular, the limits adopted by Dunoyer de Segonzac (1969) and subsequent workers at Strasbourg are too broad; their anchizone represents a range of grades of metamorphism lower than that of Kubler, widely overlapping the latter's ‘diagenetic’zone. Those adopted by some other, mainly French, authors are too narrow. The limits of the anchizone should be calibrated by inter‐laboratory standards, and the instrumental settings should be specified in full.