Other
Scientific paper
Mar 1968
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1968esrv....4...39z&link_type=abstract
Earth Science Reviews, Volume 4, p. 39-67.
Other
3
Scientific paper
Experimental determination of the phase boundaries in relevant systems, and the detailed determination of crystal structures, are together yielding increased understanding of the relationships observed among the naturally occurring pyroxenes. The relatively rigid configuration of a pyroxene chain is an important factor determining the structures which result when different cations are major constituents, and in all, six different space group symmetries have now been recognised. While the SiO chain is thus important, and is also the characterizing feature of a pyroxene according to the Bragg classification, there has perhaps been a tendency to neglect other structural features. In the case of the pyroxenes, an appreciable degree of layered character is also present, and has been given emphasis in recent publications. Close structural relationships between pyroxenes and pyroxenoids have also emerged. For nearly all of the end-member pyroxenes, details of the structure are known, but more information is needed for pyroxenes of intermediate composition, where cation ordering is a possibility. Although the well-known crystallization enrichment trends and exsolution phenomena exhibited by natural pyroxenes are in general explained by experimental phase equilibrium studies, some obscurities remain, in particular the inversion relationships between the pigeonite, hypersthene and proto-enstatite forms of the calcium-poor pyroxenes. Also, the relationship between clino-enstatite and the other MgSiO3 polymorphs has not been fully substantiated. Further evidence of the history of pyroxene crystallization can be provided by detailed studies of the distribution of cations (mainly iron and magnesium) between pyroxenes and co-existing minerals, and perhaps also by the distribution of these ions within an individual pyroxene structure. Because of the electron structure of the iron atoms, there is some tendency for these to prefer particular sites in the crystal which are generally suitable for both iron and magnesium. Contrary to this effect is that of randomisation, which is favoured by increase in temperature, so that the net result of the Mg, Fe distribution may be useful as a geothermometer. The precise configurations of atoms and their electrons (particularly for transition elements) determine the details of the absorption spectra of minerals at ultraviolet, visible and infra-red wavelengths. By such considerations, the colours and pleochroism of the pyroxenes are becoming better understood and more predictable, and conversely, observation of the absorption spectra can yield useful information about the crystal structure and distribution of cations. For pyroxenes which contain iron, Mössbauer spectroscopy can be similarly informative.
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