Enthalpies and Volumes Related to K--Na Mixing and Al-Si Order/Disorder in Alkali Feldspars

In order to investigate the thermodynamic properties of alkalifeldspars, three new feldspar ion-exchange series have beensynthesized, two based on monoclinic parent materials havingintermediate degrees of Al—Si order, the other on Amelialow albite. Acid solution calorimetric measurements have beencarried out in 20?1% HF at 50?C under isoperibolic conditionson 30 members of the three series, and compared with revisedvalues for a previously reported sanidine—analbite series.Molar volumes have been determined for all feldspars, and foran additional series based on Eifel sanidine. Enthalpies of K—Na mixing (A_ex) calculated from the 50?Cheats of solution are dependent on Al—Si distributionfor both topochemically monoclinic and triclinic alkali feldspars,and in general can be expressed as where N_Or and N_Ab are mole fractions of KAlSi₃O₈ and NaAlSi₃O₈,respectively, and Z is an ordering parameter defined as twicethe difference in the mole fraction of Al in the T1 vs the T2tetrahedral sites. A_ex values for all but the most disorderedseries are maximized toward sodic compositions, and increaseboth in magnitude and asymmetry as ordering increases. For topochemically monoclinic alkali feldspar series, volumesof K—Na mixing(V_ex) are asymmetric with N_Or, but withinthe precision of present data do not depend on Al—Si distribution: Microcline-low albite feldspars appear to have volumes of mixingwith the opposite asymmetry, but expressions of for these differ somewhat among various investigators. Since no single thermodynamic mixing property is markedly asymmetricwith respect to composition, the excess Gibbs energies impliedfrom solvus data for alkali feldspars, and maximized at sodiccompositions, are apparently the result of additive effectsof subtle asymmetries in the volumes, enthalpies, and entropiesof K—Na mixing in these minerals. The thermodynamic properties of an alkali feldspar at any compositionare significantly affected by the distribution of Al and Sibetween T1 and T2 tetrahedral sites. The enthalpy of formationat 50?C of a monoclinic potassium feldspar with perfect order(Z=1) differs by 2?19 kcal/mol from one with a completely randomAl—Si distribution (Z=0), while a value of 2?86 kcal/molapplies to analagous sodium end members. ConverselyY-ordering(between T1O andT1m sites) seems to have little or no effecton the enthalpy of formation of either end member, evidencedby the fact that most of the enthalpy differences for the lowmicrocline to sanidine and corresponding low albite to analbitetransitions (1?73 and 2?79 kcal/mol, respectively) can be attributedto Al—Si exchanges between T1 and T2 sites. Observed enthalpydifferences in alkali feldspars are probably related to strainat 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 molarvolumes of alkali feldspars.