IMMISCIBILITY IN CA-AMPHIBOLES

Abstract

The literature data of nine different occurrences of coexisting mineral pairs of Ca-amphibole have been studied and the bulk vectors, spanning the miscibility gap, derived. The additive component is always impure Mg-tremolite accompanied by some glaucophane and cummingtonite component. The four major exchange components required to describe the compositional variation in coexisting mineral pairs are the edenite (ED), tschermaks (TS), FeMg?1 and Fe3+-tschermaks (FeTs) vector. Trivalent iron is postulated on the basis of excess charges in the bulk vector the size of which coincides with residuals in Altet, ?Si, Fe and ?Mg. The four cations have equal sizes, forming the vector Fe3+ Altet Mg?1Si?1. This distribution scheme is consistent for all the different occurrences and sets the basis for a comparison. Deviations from the scheme would radically complicate the proposed exchange pattern. The ratio TS:ED in most mineral samples fluctuates between one and two. Projection of the data points in the vector space TS?ED onto the line 1ED: 2TS (Tr?Hbl) or 1ED:1TS (Tr?Prg) provides the projected tremolite content <f>$${\hbox{ X }}_{proj}^{Tr}$$</f> (= 1?XHbl or = 1?Xprg). This parameter, applied to coexisting pairs, and plotted against the ratio Mg/(Mg + Fe) shows some characteristic features about the miscibility gap. In the Mg-pure system the solvus is almost symmetric and located in the temperature range between 800 and 870°C. Small amounts (?0.10 pfu) of Fe2+ in the M(4) -sites and replacing Ca have a dramatic effect, forcing the solvus to much lower temperatures of ?650°C. An increase in the ratio Fe/(Fe + Mg) causes a shift of the solvus towards more tremolitic compositions with temperatures ?500–650°C. The maximum asymmetry of the solvus is reached where the Al-poor member (tremolite) has a composition of <f>$${\hbox{ X }}_{proj}^{Tr}$$</f>=1.0 and Mg/ (Mg + Fe) ?0.6. The corresponding Al-rich member has <f>$${\hbox{ X }}_{proj}^{Tr}$$</f>=0.5 and Mg/ (Mg + Fe) ?0.4. An anomaly of the solous is observed at Mg/ (Mg + Fe)=0.8. It manifests as a kind of highly asymmetric ‘sub-gap’ in the tremolite-rich composition range. This is explained by the partitioning of Fe2+ into the single M(3) -site and is characterized by a thermal hump to 650–700°C.