Rubidium feldspars form under low-temperature conditions in the interior zones of many pollucite-bearing rare-element granitic pegmatites. The (K-Rb)-feldspars lie close to the KAlSi 3 O 8 -RbAlSi 3 O 8 join and have up to 26.2 wt.% Rb 2 O (91 mol.% Rbf) and 1.5 wt.% Cs 2 O (3 mol.% Csf). Three subsolidus processes generate rubidium feldspars at a microscopic scale: exsolution from a primary (K, Na, Rb)-feldspar, solution-reprecipitation from the same precursor, and metasomatic coprecipitation with K-feldspar in pollucite. (i) Near-complete exsolution and phase separation of albite and (Rb, K)-feldspar are typical of a (K, Na, Rb)-feldspar precursor with up to 5 mol.%Si 4 O 8 , whereas (K, Rb)-feldspar with integral stoichiometry exsolves a Rb-enriched phase only rarely. Exsolution of the (Rb, K)-feldspar postdates that of albite because the rate of diffusion of Na is greater than that of Rb; however, migration of M-site vacancies apparently enhances rates of Rb diffusion. Rubidium feldspar coherent with microcline is ordered and triclinic. (ii) Aggregates of microporous adularian K-feldspar+(Rb,K)-feldspar are formed by solution - reprecipitation from a Rb-bearing precursor under low-temperature deuteric conditions. (iii) Adularian (Rb, K)-feldspar also coprecipitates with end-member K-feldspar (+ or -cookeite), in metasomatic reactions of hydrothermal fluid with early Rb-bearing feldspar and pollucite at approximately 300 to 150 degrees C. Such adularian feldspar typically is untwinned, monoclinic and disordered. A broad solvus must extend between K-feldspar (Or (sub approximately 100) ) and Rb-feldspar (Rbf (sub approximately 80) ), cresting below 400 degrees C. Feldspars with a heterogeneous distribution of K and Rb in the range Rbf 50 to Rbf 70 are common, but are probably metastable, preserved because of a slow rate of diffusion of Rb at low temperature. Compositional heterogeneity of the secondary (K-Rb)-feldspars on a microscopic scale also implies disequilibrium and arrested reactions.