Introductory Studies of Natural and Artificial Argillaceous Aggregates by Sound-Propagation and X-ray Diffraction Methods

Field evidence has shown wide variation in the velocities of sound waves in shales, the most abundant sedimentary-rock type. The sound velocity in and the bulk densities of a variety of natural and artificial aggregates were measured, and an attempt was made to show how mineral composition, particle orientation, and interparticle adhesion affect the velocity-density relationship. In natural samples minerals were identified by X-ray diffraction, and the degree of preferred orientation of the illite particles was measured by comparing the ratio of the intensities of the (002) to (110) lines obtained from irradiated sections cut parallel and perpendicular to the bedding. Sound-velocity measurements and X-ray results indicate that the bedding planes of flat-lying argillaceous sediments have circular symmetry for all stages of compaction and that the degree of preferred orientation parallel to the bedding plane of the basal planes of illite particles increases with increasing compaction. Large differences in sound velocities in similarly prepared drying illite and kaolinite aggregates are explained on the basis of their different fine-structure bonding. A tentative explanation is offered for the observed variation in sound velocity in pure and mixed aggregates with water content. The sound velocity in kaolinite-quartz sand mixtures apparently cannot be explained by any simple averaging of component properties but is probably intimately related to the differences in the adhesion between kaolinite particles themselves and kaolinite and sand particles. Other tentative conclusions are advanced which will require study of larger varieties of both natural and artificial aggregates for confirmation.