No abstract

SummaryAn experimental and theoretical study has been made in order to determine the number and the structure of the possible polymorphs and to determine the structural relations between them. The simplest structures are 1M, 2M1, 2M2, 3T, 20, and 6H polymorphs, and more complicated types can be developed. Some of the previously described polymorphs were not contained in the theoretical list and were re-examined. The 6M structure was found to be a 2M2 polymorph, the 6-layer triclinic type was found to be a 2M1 polymorph, and the 3M structure was shown to be a 3T type. The 24-layer triclinic structure could be described on a simpler 8-layer cell. This type together with a new 12-layer monoclinic structure, as well as other structures of higher periodicity, presumably consists of complex stacking and results from spiral-growth mechanism. Two extreme types of layer-disordered crystals may be built and a disorder of individual ions may also occur. Single stacking faults result in twinned crystals. A new twin relation (180° rotation about the [100] axis) has been recognized. Twenty specimens from extreme geological environments have been examined in order to evaluate the control of environment on the stacking. The type of stacking could not be attributed solely to the influence of pressure and temperature. Composition appears to play a dominant role in the type of stacking, and semi-quantitative structural arguments appear to support this contention. The influence of growth mechanism is discussed. A scheme for the identification of the mica polymorphs by X-ray powder and single-crystal methods is given.

In the same tetrahedron, individual Si-O (or A1-O) distances have been found to vary by amounts up to 0.1 A even when the random experimental error is less than 0.01 J~. In a single structure the differences between the mean values of the four distances in a tetrahedron are much smaller, for variations between chemically identical tetrahedra rarely amount to more than 0.02/~. The average of all the Si-O distances in a structure depends on the extent of the tetrahedral linkage, changing from 1.61 ~ in frameworks to 1.63 A in structures with isolated tetrahedra. Correspondingly, the average AI-O distance changes from 1.75 to 1.80 /~, the latter figure requiring confirmation. The mean tetrahedral distance in a feldspar structure varies linearly with percentage A1 from 1.61 for Si-O to 1.75 A for AI-O. Individual tetrahedral means for ordered structures confirm these end values. The deviations from linearity are not greater than 0.003 /~ and may merely result from random experimental error. Mean values for other framework structures deviate from the linear relation by about 0.01 A. The less accurate data for layer silicates suggest a linear relation for the overall mean Si, AI-O distance between Si-O 1.62 and AI-O 1.77 A.Estimation of the Al-content of an individual tetrahedron from the measured Si, AI-O distances must take into account the effects of structural type and of local environment of the tetrahedron. Even after correction for the structural type, it seems that the local environment may lead to errors of + 5% Al (in round figures) in addition to the effect of experimental error.It is hoped that further studies along these lines may lead to empirical relations between bond distances and atomic environment for complex structures, and ultimately to estimation of the internal energy from observed atomic coordinates.

The crystal structure of silica sodalite, including possible locations for encapsulated ethylene glycol, was determined at room temperature by using a combined single-crystal X-ray and powder neutron diffraction analysis. Unit cell composition: Siij024-2C2H4(0H)2, , = 845, cubic, Im3m, a = 8.830 (1) Á (X-ray), a = 8.8273 (1) Á (neutron). These refinements reveal that the correct space group for silica sodalite is Im3m (rather than /43m) and, therefore, that the sodalite framework is fully expanded. At room temperature, each sodalite cage contains one ethylene glycol molecule which has a range of geometrical positions. Intramolecular distances for the ethylene glycol molecule are (X-ray) C-C 1.78 (4), C-0 1.30 (6) Á, (neutron) C-C 1.70 (3), C-O 1.25 (3) Á. The shortest distances (3.4 Á) between oxygens of the framework and molecule are consistent with weak hydrogen bonding. From the neutron diffraction data it was found that a sluggish phase change from cubic to lower symmetry occurs upon cooling below 200 K. At 10 K, silica sodalite appears to be monoclinic with approximate cell parameters a = 12.250 (8) Á, b = 12.471 (8) Á, c = 8.512 (6) Á, ß = 91.37 (6)°, based on an indexing of 12 peaks, but the precise symmetry is as yet unknown.