Interpretation of Differential Thermal Curves of Mixed-layer Minerals of Illite and Montmorillonite

Abstract

MIXED-LAYER minerals of illite and montmorillonite have been known for more than a decade now, and new examples are constantly being reported¹. The identification of these minerals is based largely on X-ray examination using the tables and curves of Brown and MacEwan² or the method of MacEwan³ for random structures; the methods for ordered structures are more direct. Most workers publish differential thermal curves as supporting evidence for the presence of an inter layered structure, and in these curves there are usually two endothermic peaks in the region between 500° and 700° C, which are taken to represent loss of hydroxyl water from the separate illite and montmorillonite layers. However, a detailed examination of published and unpublished differential thermal curves of these materials shows that the sizes of the endothermic peaks in the 500°–700° C. region bear no relation to the proportion of illite to montmorillonite found by X-ray examination. Order can be attained only by dividing the curves into two series, in one of which the mala endothermic peak lies between 525° and 625° C. and in the other between 650° and 725° C. The curves in each series still bear no relation to the proportion of illite and montmorillonite, but there is an important structural difference between the groups not revealed by X-ray examination which appears to be of wide significance.