ASYMMETRY OF THE TREMOLITE-GLAUCOPHANE MISCIBILITY GAP DEFINED BY IR SPECTRA

Locating the miscibility gap between tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂ = Tr) and glaucophane (Na₂Mg₃Al₂Si₈O₂₂(OH)₂ = Gl) is an important first step in determining the phase equilibria between natural actinolite and glaucophane. Experimental investigation of this join is challenging because of the relatively low temperature (Tc ≤ 800°C) of this miscibility gap and the broad range of compositions observed in experimental re-equilibration experiments. Autocorrelation analysis offers a potentially high-resolution means of defining the relative change of the enthalpy of mixing and therefore the shape of the miscibility gap along this join.

Syntheses were done at 10 mol% increments over the range of 750°C/2.5GPa - 840°C/0.6GPa for 96 – 661 hours using reagent oxides treated hydrothermally with 4.5 wt% H₂O. Amphibole yields were > 90 wt% with minor quartz, pyroxene, and in some quenched melt. Microprobe analyses showed the amphiboles near the middle of the join deviated slight from ideal composition (enrichment in Mg and Al). Samples were analyzed at room temperature in transmission mode in both the mid- (350-5500 cm^-1) and far- (50-450 cm^-1) infrared range under vacuum with a Bruker IFS 66V spectrometer. Samples were prepared as pellets using 1:300 sample:KBr by mass for the mid-IR and 1:50 sample:polyethylene for the far-IR range. Autocorrelation analyses were done on the spectra over the ranges 100-200, 600-800, and 800-1200cm^-1.

Positive but asymmetric deviations from linearity in the δΔCorr parameters were observed across the join at all frequencies suggesting an asymmetric change in ΔHmix across the join. Attempts to extract absolute values of ΔHmix using the method of Etzel & Benisek (2008, PhysChemMin) yielded unrealistically high values. Modeling δΔCorr using either 2-parameter Margules or asymmetric formulism indicates an asymmetric gap with WTr/WGl = 1.7-2.3 or αTr/αGl = 1.4 -2.0. Assuming a critical point of 800°C and using WTr/WGl = 2.3 from autocorrelation results, one can derive a miscibility gap with WTr = 20, WGl = 8.7 kJ, and critical point composition of 0.64 Ca apfu, which is consistent with the preliminary miscibility gap proposed by Jenkins (2011, ContMinPet). No indication of ordering was observed near the middle of the join unlike the analogous augite-jadeite join (Boffa Ballaran et al. 1998, Am Min).