X-ray diffraction techniques are applicable to the study of humic materials ranging in rank from lignite to recta-anthracite. Techniques for sample preparation and analysis are described. During diagenesis and low rank metamorphism, the repeat distance of the graphite-like layers decreases progressively from 3.6-3.8 A in the lignite to high-volatile A bituminous group--to 3.5 A in the medium-volatile bituminous to anthracite group-to 3.35 A in meta-anthracite; this lattice collapse is related to the expulsion of volatiles. Also corresponding to the loss of volatiles from planar surfaces, the c aixs dimension of crystallites increases progressively from near 7 A in lignite--to near 20 A in medium to high rank coals--to 143 A in meta-anthracite. The observed changes are potentially applicable to studies of insoluble organic residues extracted from humic rich shales and other normal sediments undergoing diagenesis and low rank metamorphism.

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