Structural and thermodynamic properties of Ca-Al-bearing amphiboles

Abstract

Structural and thermodynamic properties of Ca-Al bearing amphiboles“ The Ca-Sr and Mg-tschermaks (MgSi ↔ AlAl) substitutions in tremolite, and the Ca-Sr substitution in diopside were experimentally investigated. Syntheses were performed in the presence of a halogenidic fluid. Experiments about Ca-Sr substitution were carried out at 750°C and 200 MPa and experiments about Mg-tschermaks substitution were carried out at 600-850°C and 200-2000 MPa. Both tremolite and diopside structures are flexible for a complete substitution of Sr for Ca. A continuous solid solution series exists between tremolite and Sr-tremolite. (Ca,Sr)-diopside solid solutions could be synthesized between XSr = 0.00-0.31 and 0.90-1.00. Along the Mg-tschermaks vector solid solutions between tremolite and magnesiohornblende were observed. The lattice parameters of tremolite and diopside are a linear function of the Sr-content. For (Ca,Sr)-tremolites the lattice parameters a, b and s increase with increasing Sr-content. For (Ca,Sr)-diopsides a and b increase and s decreases with rising Sr-content. Increasing Mg-tschermaks content in tremolite also changes the lattice parameters linearly, a and b decrease whereas c and s increase. Complex fine structures of the OH stretching bands can be observed in IR spectra of (Ca,Sr)tremolites and Al-tremolites. Fine structures were assigned to substitutions of Sr, Ca and Mg on M4 sites in (Ca,Sr)-tremolite, and to Mg-Al substitution on M2 and M3 sites and to Si-Al substitution on T1 sites in Al-tremolite. The assignments were supported by quantitative calculations, which indicate a statistical distribution of Sr, Ca and Mg on M4 sites and Mg and Al on M2 and M3 sites in tremolite. The distribution of Sr and Ca between tremolite, diopside and chloridic solution were investigated at 750°C and 200 MPa. Sr fractionates strongly into the fluid. The mineral/fluid partition coefficients for Sr are DSr amphibole/ fluid = 0.045 and DSr pyroxene/ fluid = 0.082. The mixing energies of (Ca,Sr)-tremolite and (Ca,Sr)-diopside series were calculated using a regular solution model. Interaction parameters of WCaSr amph = 9.8 kJ and WCaSr pyr = 11.7 kJ were derived. The thermodynamic properties of the tschermakite endmember and the mixing properties along the tremolite-tschermakite join were extracted from six exchange reactions. Ideal mixing models were tested for Mg-tschermaks substitution along the tremolite-tschermakite join. Best fits were obtained for a two-site coupled model, resulting in an enthalpy of formation of ∆ f H ts o = -12528.3 ± 11.7 kJ/mol and a standard entropy of Sts o = 556.5 ± 12.0 J/mol/K for tschermakite endmember.