Zeolites having the structures of phillipsite, merlinoite, and gobbinsite were synthesized from clear solutions at 80°C in the system Na2O-K2O-Al2O3-SiO2-H2O and their morphologies, cell parameters, and compositions determined. At 3.5 M silica concentration, the formation of merlinoite (synthetic zeolite W) is favored over the formation of phillipsite (synthetic zeolite ZK-19) by solution conditions of high pH (> 13.6) and low Na/(Na + K) ratios (<0.5).

Using the information obtained from the synthesis experiments, the presence of merlinoite was predicted in sediments from Searles Lake, a saline, alkaline lake in California with ideal physiochemical conditions for its formation. Merlinoite was subsequently discovered to occur in tuffaceous sediments as part of an authigenic silicate zonation pattern from phillipsite → phillipsite + merlinoite → merlinoite → K-feldspar with increasing depth. Because of the close similarities in the physical properties of phillipsite and merlinoite, merlinoite may be much more common as an authigenic mineral than is currently realized.

To understand and predict the effects of a thermal pulse induced by a radioactive waste repository on clinoptilolite-bearing rocks, the lattice parameters of 6 natural and 3 cation-exchanged (Ca, K, Na) clinoptilolites were studied as a function of temperature. The samples were examined at room temperature, under vacuum, and at 50°C increments to 300°C using a high-temperature X-ray powder diffractometer. The unit cell of all samples decreased in volume between 20° and 300°C Na-saturated clinoptilolite underwent the greatest volume decrease (8.4%) and K-saturated clinoptilolite the smallest (1.6%), of the clinoptilolites studied. The volume decrease for the Ca-saturated clinoptilolite was 3.6%. The highest percentage decrease for each sample was along the b axis, generally 80–90% of the total volume decrease. The change in the a axis was the smallest and was usually <5%, although 26.5% of the contraction of the Na-exchanged clinoptilolite was along a. The bulk of the volume contraction of many samples occurred on evacuation at room temperature, demonstrating that the observed changes were due to water loss and not to temperature-induced structural changes. Low-angle scattering was significantly reduced upon evacuation for every sample, and the 110 reflection of clinoptilolite at 7.45°2θ became obvious, whereas it was not in the untreated samples.

These data show that the effects of heating on the unit-cell volume of clinoptilolite depend strongly on the exchangeable cation content. Significant reductions in the unit-cell volumes of natural, mixed Na-K-Ca clinoptilolites could take place in rocks in a repository environment, particularly if the clinoptilolites occurred in unsaturated, dehydrated rock. The unit-cell volumes of clinoptilolites in partially saturated rocks at temperatures below 100°C, however, should not decrease significantly.

A procedure leading to the systematic derivation of all maximum-degree-of-order (MDO) polytypes in the three mica families, based on the order-disorder (OD) interpretation of mica structures, has been developed. There are 6 non-equivalent (8 non-congruent) homo-octahedral, 14 (22) mesooctahedral, and 36 (60) hetero-octahedral MDO mica polytypes. The ideal ditrigonalization of tetrahedral sheets leads to a splitting of the polytypes in any mica family into two subfamilies with octahedral and trigonal prismatic coordination of the interlayer cations, respectively.

The maximum-degree-of-order (MDO) polytypes in all three mica families have been classified into two subfamilies and five homomorphous MDO groups according to their superposition structure and the YZ projection of their structure, respectively. This classification, which is closely related to the diffraction pattern of micas, can be derived directly from the fully descriptive polytype symbols and facilitates the calculation of the identification diagrams as well as the recognition of the relations of homomorphy between the three mica families. All mica structures refined so far by the least-squares method according to the present standards, are those of MDO polytypes.

Synthetic Al-hematites prepared from ferrihydrites, at low (∼ 100°C) and high (400° and 800°C) temperatures were studied for their morphological, crystallochemical, and infrared (IR) characteristics. Low-temperature Al-hematites had a platy morphology (the plate thickness was inversely related to amount of Al substitution), and the high temperature Al-hematites showed a poorly defined morphology due to interparticle sintering. In the low-temperature Al-hematites shifts in the IR mode frequencies were noted and could be partly explained by a shape factor that was deduced from particle morphology. The intrinsic effect of Al substitution, however, was to produce shifts of as much as 10–15 cm−1 for the highest Al substitution (∼ 16%). Similar shifts were observed for the high-temperature hematites in which morphology was not appreciably affected by Al substitution.