Construction of thermodynamic stability diagrams involving dioctahedral potassium clay minerals

Abstract

Some of the most important alteration products of rock-water interaction, e.g. in geothermal systems, are dioctahedral clay minerals such as illites, phengites and montmorillonites. By showing that most of these minerals are adequately described in terms of solid solutions involving the three end-member components mica, pyrophyllite and celadonite, their general compositions may be given as: Y m+cAl 2−c(Mg,Fe 2+) cAl mSi 4−mO 10(OH) 2 where Y m + c is the sum of cation equivalents on the exchange sites; c the mole fraction of Mg 2+ and Fe 2+ on octahedral sites; and m the mole fraction of Al 3+ on tetrahedral sites. By use of an ideal mixing model of atoms on homological sites the activities of the end-members pyrophyllite and muscovite may be evaluated according to, respectively: ppyrophyllite = (1−m−c)[ (2−c) 2 ] 2[( (4−m) 4 ] 4 and amuscovite = xK (m+c)[ (2−c) 2 ] 2m[( (4−m) 3 ] 3 and On the basis of reported analyses, the dioctahedral clay minerals were classified in terms of their mica and pyrophyllite activities allowing the stability ranges of illites, beidellites, phengites, montmorillonites and celadonites to be presented in terms of two- and three-dimensional diagrams involving the parameters: K H = log(α K +/α H +, Al H = log (α Al 3+/α 3 H +), and s = log α SiO 2(aq)