Issue 3: 1996

Abstract

Immiscibility in Ca-amphiboles

AJ. Zingg Montan University, Leoben, Geoscience Institute, A-8700 Leoben, Austria and Present address: Feldeggstrasse 3, 8645 Jona, Switzerland
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), tschermak's (TS), FeMg-1 and Fe³⁺-tschermak's (FeTs) vector. Trivalent iron is postulated on the basis of excess charges in the bulk vector the size of which coincides with residuals in Al^tet, -Si, -Fe and -Mg. The four cations have equal sizes, forming the vector Fe³⁺ Al^tetMg-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 X^(Tr,proj) (=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^oC. 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^oC. An increase in the ratio Fe/(Fe+Mg) causes a shift of the solvus towards more tremolitic compositions with temperatures ~500-650^oC. The maximum asymmetry of the solvus is reached where the Al-poor member (tremolite) has a composition of X^(Tr,proj)=1.0 and Mg/(Mg+Fe) ~0.6. The corresponding Al-rich member has X^(Tr,proj)=0.5 and Mg/(Mg+Fe) ~0.4. An anomaly of the solvus 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^oC.

Pages: 471 - 496

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