Chapter 4. Structures and microstructures of non-classical pyriboles
by Krassimir N. Bozhilov
Non-classical pyriboles (NCP) represent a complex group of related mineral phases that range structurally and compositionally between pyroxenes and mica. They are characterized by tetrahedral silicate chain multiplicity other than single or double, and typically occur as polysomatic lamellar intergrowths between themselves and/or with pyroxenes and amphiboles in diverse geological environments including regional and contact-metamorphosed ultramafic, mafic and ocean-floor rocks and hydrothermally altered sediments. The study of fine lamellar aggregates of NCPs is complicated further by the presence of extensive and diverse microstructures such as stacking and chain multiplicity faults, dislocations, chain terminations, and polysomatic and polytypic disorder.
Proper characterization of pyriboles necessitates utilization of local atomic-scale resolution techniques such as electron microscopy, including high-resolution transmission electron microscopy (HRTEM), electron diffraction and microanalysis. This chapter reviews the major concepts that govern the structural and crystalchemical properties of NCP following the principles introduced by Thompson (1970, 1978). Summary information is assembled here about the progress that has been made over the past 20 years regarding the structures and microstructures, modes of occurrence, methods of characterization and synthesis of NCP. Further, the role of polysomatic disorder in the extent and interpretation of solid solution among pyribole endmembers is assessed critically.
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