Summary
Three types of late, subsolidus (K-Rb)-feldspars occur in the spodumene-subtype, complex-type, pollucite-bearing Rubellite rare-element pegmatite at Lilypad Lakes, near Fort Hope in northwestern Ontario: (1) Exsolution followed by hydrothermal reconstitution generated diffuse bands, patches and veinlets of Rb-enriched (< 43 mol. % Rbf) and Rb-depleted triclinic K-feldspar in black blocky microcline; (2) metasomatism of pollucite produced aggregates of adularia which range from 14 to 73 mol.% Rbf, rimmed by pure 100 mol.% Kfs feldspar, and are most likely highly disordered; (3) blocky K-feldspar was replaced by salmon-coloured, fine-grained, Na-poor and Rb-free maximum microcline with a composition close to 100 mol.% Kfs, associated with calcite and fluorite. After initial solid-state unmixing in blocky microcline, the reconstituted and metasomatic (K-Rb)-feldspars of the first two types could have formed simultaneously, under the influence of the same aqueous fluid. The exsolved and reconstituted Rb-rich feldspar inherited the (Al,Si)-ordered framework of the blocky precursor, whereas metasomatic neocrystallization generated metastable disordered (K-Rb)-adularia. In contrast, a different, F- and CO3-rich fluid produced the metasomatic salmon-coloured K-feldspar, and promoted its perfect (Al,Si)-order.
Zusammenfassung
In dem Pollucit and seltene Elemente führenden Rubellit-Pegmatit (Spodumen-Subtyp, komplexer Typ) bei Lilypad Lakes, nahe Fort Hope in Nordwest-Ontario kommen drei Typen von späten (K,Rb)-Feldspäten vor: (1) Entmischung gefolgt von hydrothermaler Rekonstitution schuf diffuse Bänder, Flecken and Gdngchen von an Rb angereicherten (< 43 Mol-% Rbf) and an Rb verarmtem triklinem K-Feldspat in schwarzem, blockigem Mikroklin; (2) Pollucit-Metasomatismus führte zur Bildung von Adular-Aggregaten mit von 14 bis 73 Mol-% Rbf, der von reinem 100 Mol-% Kfs gesäumt and höchst-wahrscheinlich stark ungeordnet ist; (3) blockiger K-Feldspat wurde durch lachsfarbigen, feinkömigen, Na-armen and Rb-freien maximalen Mikroklin mit einer Zusammensetzung nahe 100 Mol-% Kfs ersetzt, der mit Calcit and Fluorit vergesellschaftet ist. Nach der ursprünglichen Festkörperentmischung in blockigen Mikroklin konnten sich die neugebildeten und metasomatischen (K,Rb)-Feldspäte der beiden ersten Typen gleichzeitig and unter dem Einfluß des gleichen wäßrigen Fluides gebildet haben. Die entmischten and neugebildeten Rb-reichen Feläspdte erbten die (Al,Si)-Ordnung des Gerüstes des blockigen Vorläufers, während die metasomatische Neokristallisation metastabil ungeordnete (K,Rb)-Adulare bildete. Im Gegensatz dazu bildete ein F- und CO3-reiches Fluid den metasomatischen, lachsfarbigen K-Feldspat und verursachte seine vollständige (Al,Si)-Ordnung.
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Teertstra, D.K., Černý, P. & Hawthorne, F.C. Geochemistry and petrology of late K-and Rb-feldspars in the Rubellite pegmatite, Lilypad Lakes, NW Ontario. Mineralogy and Petrology 65, 237–247 (1999). https://doi.org/10.1007/BF01161962
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DOI: https://doi.org/10.1007/BF01161962