Velocities of Compressional and Shear Waves in Zeolites: Heulandite and Stilbite

In recent years, an increasing number of sedimentary rocks have been reported to contain zeolite minerals among their constituents. Due to their lower densities, zeolites interfere with the density log; hence, whenever their presence is recognized, proper corrections are made in the evaluation of porosity from the density log. Study of a similar question about the effect of zeolites in the evaluation of porosity from acoustic log measurements requires a knowledge of elastic wave velocities in zeolites. A literature search to find data on elastic wave velocities in zeolites proved fruitless; however, we were able to obtain two hand specimens of zeolites from Iceland. Through X-ray analysis and from optical properties, they were identified as heulandite and stilbite. The specimens were fragile; therefore, for convenience in handling during the measurements, they were cast in clear plastic. Each was machined to expose two opposite and parallel surfaces of about 2.5 cm in diameter in a convenient direction (Z), since the crystal axes could not be identified. In this manner, a heulandite sample of 13.828-mm thickness and a stilbite sample of 22.327-mm thickness were obtained for velocity measurements. To measure velocities of the compressional waves, these samples were placed between a pair of compressional wave transducers (280 kHz resonant frequency) in contact with the exposed surfaces. The travel times of the compressional wave pulses in the Z direction were measured by the ultrasonic measurement apparatus described in Ref. 2. The compressional velocities, obtained within 0.5 percent, are listed in Table 1. Heulandite and stilbite are known to have monoclinic structures;, therefore, they are expected to have velocity anisotropy, Since the samples were cast in epoxy, we could not obtain velocity information in the directions other than Z due to problems in mechanical handling of the samples. We were able, however, to obtain indications of anisotropy by changing orientation of the vibration direction of the shear waves.

P. 1099