Rubidium-rich feldspars and associated minerals from the Luolamaki pegmatite, Somero, Finland

Abstract

Rubidium feldspar occurs near the core zone of the highly fractionated petalite-subtype Luolamaki granitic pegmatite in intimate intergrowth with other feldspars which are part of a characteristic sequence of alteration of pollucite. Pods of pollucite are cut by 5-20 cm-wide veins of albite, petalite, non-perthitic microcline, lepidolite and quartz, by thinner veins of fine-grained micas and spodumene, and are replaced by metasomatic adularia. Grains of rubidium feldspar occur as a potentially ordered phase in the vein microcline in association with earlier-exsolved albite, and also as late thin (< 5 μm) veinlets. Rubidium feldspar also occurs as a potentially disordered phase which crystallized along with metasomatic adularia. Both generations of (Rb,K)-feldspar have a similar compositional range, close to the join KAlSi3O8-RbAlSi3O8, typically with up to ~21 wt.% Rb2O (~70 mol.% Rbf) and with minor Cs, but neglible Na, Ca, Fe or P. Extreme compositions have 26.0 wt.% Rb2O (89.0 mol.% Rbf) and 1.26 wt.% Cs2O (2.8 mol.% Csf). The diffuse compositional gradients from microcline to rubicline are consistent with a solid-state exsolution origin, followed by fluid-assisted textural coarsening which generates distinct phase boundaries. In contrast, metasomatic adularian (Rb,K)-feldspar was precipitated at low temperature (250-150°C) and fine-scale zoning with variable K/Rb is preserved as a growth feature.