Illite a N D Smectite: Metastable, Stable or Unstable? Further Discussion a N D a Correction

Essene and Peacor (1995 = " E P " ) published a general review of clay mineral thermometry, the stability of clay minerals and the relevance to natural clay mineral assemblages. Some of the conclusions in that paper have been criticized by Aja and Rosenberg (1996 = "AR") . Our paper dealt with some issues about which there has been healthy (and sometimes highly charged) debate. Such dialogue normally takes the form of 2 papers frequently published in tandem, one a criticism and another a response. We are grateful for a belated opportunity to further discuss the thermodynamic and mineralogical status of compositionally and structurally variable clay minerals, in part because this subject is of critical importance to clay mineralogists. It is therefore essential that the basic theory be clearly defined and understood. This has largely to do with some elementary, but critically important, thermodynamic relations. The discussion of issues by AR is largely in error or misleading and begs for correction. Although Aja and Rosenberg (1992) is not specifically addressed here, many of the points raised represent a criticism of that assessment as well. We will respond only to some of the more important points of disagreement below.