Structural Conversions in Crystalline Systems and their Importance for Geological Problems

Publisher Website
Google Scholar
Abstract
From G. Tammann’s theory of the structure of crystalline solutions, the discussion of disordered atomic distributions in apparently homogeneous crystalline phases has developed to an important branch of investigation in metallurgy and mineralogy, especially in the study of ores and rock-forming mineral systems. Ordered modifications with superstructures, characterized by forbidden lines and interference spots in X-ray diffraction patterns, are compared with disordered modifications of the same chemical composition but which have a higher symmetry and a higher entropy. The transition between both modification types either may be gradual or may be characterized by a critical temperature of order ⇄ disorder arrangements (inversions of the second class). Intermediate modifications with characteristic physical properties may occur. The experimental methods suitable for the systematic study of structural conversions are illustrated by a selection of examples taken from simple metal alloys, sulfides, and related ore minerals, halides, oxides, oxysalts, and silicates, including aluminosilicates. The special discussion of the complicated structural conversion phenomena of alkali and plagioclase feldspars makes possible, by detailed structural studies, important conclusions on their genetic conditions in nature and on geological thermometry. Further information on the important role which structural conversions play in the group of the silica minerals, especially for the crystallization of cristobalite and tridymite, is given. The conversions of those rock-forming mineral systems which structurally are characterized as derivatives of the silica modifications, namely eucryptite, nepheline, kalsilite, kaliophilite, belong in the same group of phenomena. For a wider understanding of structural conversions the polytypism in the layer structures of the mica and the chloriteclay mineral groups, to which belong also the highly interesting interstratification structures of the vermiculites, is discussed. Even in the solution of such problems as the constitution of the alites in Portland cement clinkers, the similar structural characteristics of defects and conversions, as well as the important double compounds of silicates and nonsilicates in which coupled cation replacements occur, can be considered. The monograph concludes with a special discussion of the double carbonates and their importance for the explanation of the calcitemagnesite system; dolomite is the intermediate compound, and its characteristic chemical reactions are highly important for the theory of the formation of sedimentary carbonate rocks and their metamorphic changes.

This publication has 0 references indexed in Scilit: