Effect of amount of material used on sedimentation slides in the control of illite "crystallinity" measurements

Tests are reported on the effect of the amount of sample used on three sets of sedimentation slides of pelitic samples from the Palombini shale, N Apennines, Italy, which has been subjected to conditions extending from late-stage diagenesis to low-grade metamorphism. The illite crystallinity indices (IC) are strongly dependent on the amount of sample material when 'thin' slides are used (loading < 0.5 mg/cm (super 2) ). For sedimentation slides with a loading > 0.5 mg/cm (super 2) , the variation of the IC value with the weight of the clay layer is much less marked, but not negligible. In a plot of crystallinity index (Kubler index) vs sample loading, the representative points define straight lines with a moderate slope; samples with different 'crystallinities' are arranged on subparallel lines. A procedure is suggested to normalize the IC values to a selected reference amount of material.