Sub solidus relations in the plagioclase feldspar

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

Almost half a century ago Bowen (1913) determined the liquidus-solidus relationships across the plagioclase feldspars series. He showed that, at !east at high temperatures, a simple solid solution series probably existed over the whole range of compositions from pure albite to pure anorthite. His work was taken to confirm the view that this simple replacement series extended through the sub-solidus at all temperatures. and for about another twenty five years this assumption was not really challenged. However evidence began to accumulate in the period before the war which suggested that feldspar relationships were not so simple. An increasing number of reliable measurements showed that the optics of a feldspar of a given composition may depend on its previous geological history. The detailed and careful work of Spencer using both natural and heat treated alkali feldspars had begun to elucidate the puzzling mineralogical relationships in this series, and lent weight to the view that there were polymorphic transformations at least for the pobtssium-rich feldspars, and possibly for other feldspars as well. At about same time Taylor and his coworkers ( 1933. 1934) published the results of the first structure determinations of feldspars, which confirmed Machatschki's suggestions that all feldspars had a framework structure of linked SiO4 or AlO4 tetrahedral groups, and that the more obvious crystallographic differences between the alkali and plagioclase feldspars were due to the sizes of the cations which occupied interstices in the negatively-charged framework. It was then suggested by Barth (1934) that some of the polymorphic relationships could be linked to the distribution of Si and Al atoms within the tetrahedral framework, and so the basis was laid for the Si-Al order-disorder conception which has proved fundamental to all feldspar polymorphism.

The first direct evidence that the plagioclases did not show simple replacement over the whole series came from Taylor, Darbyshire and Strunz (1934); later Chao and Taylor (1940) demonstrated that three different structural regions could be recognised. Since that time it has been found that these original structural regions are more complex than was suspected by Chao and Taylor, and in the last decade a considerable volume of work has attempted to elucidate the sub-solidus of the plagioclases. It is the purpose of this paper to review the present position of our knowledge.