Download PDF by R. Ashenhurst: Acm Turning Award Lectures: The First Twenty Years : 1966 to

By R. Ashenhurst

ISBN-10: 0201077949

ISBN-13: 9780201077940

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Extra info for Acm Turning Award Lectures: The First Twenty Years : 1966 to 1985

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It is frequently observed that the various members of groups of minerals with comparable structural characteristics are brought about by what is called diadochy. Diadochy describes the replacement or replaceability of one atom or ion in a crystal lattice by another (see the A. G. I. 80). It is known, for example, that in certain micas Fez+ can be replaced by Mg2+;in other minerals Al3+ can be replaced by Fe3+ or by Fez+. In amphiboles and pyroxenes the substitution of Si by Al, linked with the substitution or addition of ions that can restore electrostatic neutrality, is a well-known aspect of these mineral groups.

In short, such mechanisms generally consist of the following stages: ( I ) plagioclase is changed into albite by a replacement of Ca by Na; (2) the albite thus formed is partly replaced by K-feldspar; (3) the organization in space of Na-rich and K-rich phases after such a process of replacement may show either the same or a similar pattern such as the one observed after an exsolution of a homogeneous alkali feldspar phase.

Other investigations prove that in the light of geological circumstances it is highly probable that alkali feldspars originally of monoclinic symmetry, may well become triclinic after “cooling”. The presence of stress conditions, widespread within the earth’s crust, is assumed to trigger these changes under the influences of elevated temperatures and rather high partial pressures of HzO. Moreover, hightemperature experiments give a clue as to the origin and further development of the various alkali feldspars occurring in igneous and metamorphic rocks.

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Acm Turning Award Lectures: The First Twenty Years : 1966 to 1985 by R. Ashenhurst

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