Immiscibility in Ca-amphiboles |
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Authors: | ZINGG A J |
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Institution: | MONTAN UNIVERSITY, LEOBEN, GEOSCIENCE INSTITUTE A-8700 LEOBEN, AUSTRIA |
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Abstract: | The literature data of nine different occurrences of coexistingmineral pairs of Ca-amphibole have been studied and the bulkvectors, spanning the miscibility gap, derived. The additivecomponent is always impure Mg-tremolite accompanied by someglaucophane and cummingtonite component. The four major exchangecomponents required to describe the compositional variationin coexisting mineral pairs are the edenite (ED), tschermak's(TS), FeMg1 and Fe3+-tschermak's (FeTs) vector. Trivalentiron is postulated on the basis of excess charges in the bulkvector the size of which coincides with residuals in Altet,Si, Fe and Mg. The four cations have equal sizes,forming the vector Fe3+ Altet Mg1Si1. This distributionscheme is consistent for all the different occurrences and setsthe basis for a comparison. Deviations from the scheme wouldradically complicate the proposed exchange pattern. The ratioTS:ED in most mineral samples fluctuates between one and two.Projection of the data points in the vector space TSEDonto the line 1ED: 2TS (TrHbl) or 1ED:1TS (TrPrg)provides the projected tremolite content (= 1XHbl or = 1Xprg). This parameter,applied to coexisting pairs, and plotted against the ratio Mg/(Mg+ Fe) shows some characteristic features about the miscibilitygap. In the Mg-pure system the solvus is almost symmetric andlocated in the temperature range between 800 and 870C. Smallamounts (0.10 pfu) of Fe2+ in the M(4) -sites and replacingCa have a dramatic effect, forcing the solvus to much lowertemperatures of 650C. An increase in the ratio Fe/(Fe + Mg)causes a shift of the solvus towards more tremolitic compositionswith temperatures 500650C. The maximum asymmetry ofthe solvus is reached where the Al-poor member (tremolite) hasa composition of =1.0 and Mg/ (Mg + Fe) 0.6. The corresponding Al-rich member has =0.5 and Mg/ (Mg + Fe) 0.4. An anomalyof the solous is observed at Mg/ (Mg + Fe)=0.8. It manifestsas a kind of highly asymmetric sub-gap in thetremolite-rich composition range. This is explained by the partitioningof Fe2+ into the single M(3) -site and is characterized by athermal hump to 650700C. KEY WORDS: tremolite; hornblende; pargasite; immiscibility; solous |
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