| Popis: |
Previous work concerning the structure of chrysotile is first reviewed, to provide a basis for discussion of the natures of two of the less common varieties of chrysotile - Povlen-type chrysotile and para-chrysotile. The results of x-ray diffraction, electron diffraction and electron-optical experiments on Povlen-type chrysotile are presented. On the basis of these observations a lath-like model for this material is rejected. In the course of discussion of the diffraction effects arising from Povlen-type chrysotile, the earlier theoretical treatment of the diffuse reflections of chrysotile is extended. In particular, the effects on the profiles of these reflections of a variety of distributions of fibril diameters within a fibre bundle have been investigated. The character of the reflections to be expected from chrysotile, based upon a high-order helical structure are described in the course of the evaluation of a possible helical model for Povlen-type chrysotile. This model is, however, rejected in favour of a polygonal model, which accounts satisfactorily for the observed diffraction effects. An explanation is offered for the "lumpiness" of the diffuse reflections apparent in some x-ray fibre photographs of chrysotile. After a review of the few previous observations of para-chrysotile the results of electron-optical and electron diffraction studies are presented. An attempt is made to interpret these results in terms of a possible structural model for para-chrysotile. As in the case of Povlen-type chrysotile a lath-like model is rejected. Computations of the profiles of some of the diffuse reflections of para-chrysotile (based upon a cylindrical structure), which have not previously been published, are reported. In the light of the results of these calculations, which were carried out both for fibres consisting of bundles of fibrils exhibiting various distributions of diameters (applicable to x-ray diffraction) and for the case of single fibrils of various diameters (applicable to electron diffraction), a cylindrical model is considered to be inadequate to explain the present diffraction results. It is suggested that a polygonal model, similar to that proposed for Povlen-type chrysotile, might be more appropriate. Computer programs have been written to carry out the computation of the reflection profiles discussed in these two chapters, and are given in an Appendix. In the succeeding chapters observations concerning several commonly occurring accessory minerals of serpentinites are reported. X-ray diffraction and electron diffraction studies of fibrous brucite are described. The intergrowth of brucite with chrysotile, particularly with para-chrysotile, is discussed in the light of the present observations in the electron microscope. The nature of 'nemalite' is discussed and a model, based upon intergrowth with chrysotile, is proposed to account for the published accounts of this material. The structural and chemical characteristics of some of the hydroxycarbonate minerals are reviewed. The stability of brucite in the weathering environment and some chemical and crystallographic aspects of its alteration to various hydroxycarbonate-type minerals are considered. Observations on the textural development of magnetite in serpentinites are reported. Some chemical data are presented, together with the results of an x-ray investigation into the structural orientation of fibrous magnetite intergrown with other minerals in cross-cutting veins. Finally, an attempt is made to relate some of the observations reported in the present study to recent discussion of the processes involved in serpentinisation. |
