Winchite and the actinolite–glaucophane miscibility gap

Abstract

Leake et al.¹ have recently emphasized the apparent scarcity of winchite s.l., NaCa(Mg, Fe²⁺)₄(Al, Fe³⁺) [Si₈O₂₂/(OH)₂] (ref. 2), an amphibole intermediate in composition between the glaucophane–riebeckite series of high-pressure blueschists and the tremolite–actinolite series of lower-pressure greenschists. The rarity of winchite is considered consistent with the existence of a miscibility gap^3,4 between these two amphibole series at the metamorphic P–T conditions normally attained in nature. Here we present optical and X-ray properties of Mg- and Al-rich winchite from Venezuela and draw attention to several other recent winchite analyses. In fact, as Schliestedt's⁵ new data on co-existing sodic and calcic amphiboles in blueschists show, only the present Mg–Al-rich winchite definitely lies within the above miscibility gap.