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 (≤ 43mol.% Rbf) and Rb-depleted triclinic K-feldspar in black blocky
... [Show full abstract] 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.
