Experimental and theoretical studies of the mica polymorphs

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.

Activity/composition relations among silicates and aqueous solutions; I, Thermodynamics of intrasite mixing and substitutional order/disorder in minerals

“…The observations summarized above are consistent with conclusions reached from structure refinements of layer silicates by Smith and Yoder (1956), Zvyagin (1967), and others. Although Bailey (1975) questioned the validity of using ideal space groups in these structure refinements (and suggested that tetrahedral order may be more prevalent than previous studies indicate), it nevertheless appears reasonable to assume on the basis of all the various data currently available that in most cases tetrahedral disorder prevails in low-temperature layer silicates other than margarite and rare polytypes such as phengite 2M2 and muscovite 3T.…”

Periodic intensity distribution (PID) of mica polytypes: symbolism, structural model orientation and axial settings

“…Unlike Z vectors, these vectors do not pass through any cation site in the O sheet. Therefore, stacking vectors of Smith & Yoder (1956) cannot distinguish between M1 and M2 layers and they too correspond to the homo-octahedral approximation.³ Both the M layer and the TS unit layers have the origin in the interlayer region. However, TS unit layers are shifted along [001] by half the thickness of the M layer ( Fig.…”

“…The stacking sequence of mica polytypes is quite often described by means of the stacking vectors according to the original de®nition of Smith & Yoder (1956), i.e. the (001) projection of vectors connecting two nearest interlayer cations on the two sides of a layer (Fig.…”

“…1 and 2) with interlayer cations between adjacent layers. The T±O±T layer is commonly considered as a cleavage unit (Kogure, 1997) and polytypism arises from the stacking of this layer with n  60 rotations (0 n 5) (Smith & Yoder, 1956). Polytypes in which the position of any layer relative to the others and the transition from it to the adjacent layers are the same or equivalent for all layers are called homogeneous polytypes .…”

“…Polytypes in which the position of any layer relative to the others and the transition from it to the adjacent layers are the same or equivalent for all layers are called homogeneous polytypes . The homogeneous polytypes were ®rst derived by Smith & Yoder (1956), who called them`simple polymorphs'. By using Ramsdell (1947) notation, these polytypes are identi®ed as 1M, 2M 1 , 3T, 2M 2 , 2O and 6H [symbols according to Guinier et al (1984)].…”