ENTHALPIES AND VOLUMES RELATED TO K--NA MIXING AND AL-SI ORDER/DISORDER IN ALKALI FELDSPARS

Abstract

In order to investigate the thermodynamic properties of alkali feldspars, three new feldspar ion-exchange series have been synthesized, two based on monoclinic parent materials having intermediate degrees of Al—Si order, the other on Amelia low albite. Acid solution calorimetric measurements have been carried out in 20�1% HF at 50�C under isoperibolic conditions on 30 members of the three series, and compared with revised values for a previously reported sanidine—analbite series. Molar volumes have been determined for all feldspars, and for an additional series based on Eifel sanidine. Enthalpies of K—Na mixing ( A ex ) calculated from the 50�C heats of solution are dependent on Al—Si distribution for both topochemically monoclinic and triclinic alkali feldspars, and in general can be expressed as $$\begin{array}{c}{A}_{ex}\left(\hbox{ kcal }/\hbox{ mol }\right)=\left(3.10+15.56Z-7.39{Z}^{2}\right){N}_{Or}{N}_{Ab}^{2}\\ +\left(4.95-2.33Z\right){N}_{Ab}{N}_{Or}^{2}\end{array}$$ where N Or and N Ab are mole fractions of KAlSi 3 O 8 and NaAlSi 3 O 8 , respectively, and Z is an ordering parameter defined as twice the difference in the mole fraction of Al in the T1 vs the T2 tetrahedral sites. A ex values for all but the most disordered series are maximized toward sodic compositions, and increase both in magnitude and asymmetry as ordering increases. For topochemically monoclinic alkali feldspar series, volumes of K—Na mixing( V ex ) are asymmetric with N Or , but within the precision of present data do not depend on Al—Si distribution: $${V}_{ex}\left(cal/b-mol\right)=0.110{N}_{Or}{N}_{Ab}^{2}+0.078{N}_{Ab}{N}_{Or}^{2}.$$ Microcline-low albite feldspars appear to have volumes of mixing with the opposite asymmetry, but expressions of $${\stackrel{\¯}{V}}_{ex}$$ for these differ somewhat among various investigators. Since no single thermodynamic mixing property is markedly asymmetric with respect to composition, the excess Gibbs energies implied from solvus data for alkali feldspars, and maximized at sodic compositions, are apparently the result of additive effects of subtle asymmetries in the volumes, enthalpies, and entropies of K—Na mixing in these minerals. The thermodynamic properties of an alkali feldspar at any composition are significantly affected by the distribution of Al and Si between T1 and T2 tetrahedral sites. The enthalpy of formation at 50�C of a monoclinic potassium feldspar with perfect order (Z=1) differs by 2�19 kcal/mol from one with a completely random Al—Si distribution (Z=0), while a value of 2�86 kcal/mol applies to analagous sodium end members. Conversely Y -ordering (between T1O andT1m sites) seems to have little or no effect on the enthalpy of formation of either end member, evidenced by the fact that most of the enthalpy differences for the low microcline to sanidine and corresponding low albite to analbite transitions (1�73 and 2�79 kcal/mol, respectively) can be attributed to Al—Si exchanges between T1 and T2 sites. Observed enthalpy differences in alkali feldspars are probably related to strain at domain boundaries, whether the domains are extremely small, or somewhat larger as in modulated structures. Neither Z -nor Y -ordering has a substantial effect on the molar volumes of alkali feldspars.