First natural occurrences of end-member pollucite: A product of low-temperature reequilibration
Teertstra, David K.; Černý, Petr
European Journal of Mineralogy Volume 7 Number 5 (1995), p. 1137 - 1148
published: Oct 5, 1995
manuscript accepted: Apr 3, 1995
manuscript received: Apr 26, 1994
ArtNo. ESP147050705009, Price: 29.00 €
Abstract Sodium-poor pollucite extremely close to the ideal end-member composition CsAlSi2O6 was discovered on a microscopic scale in five specimens from localities in Europe, Africa and North America. It crystallized at subsolidus temperatures of about 300-200 °C under hydrothermal conditions and is closely associated with the latest minerals in a typical sequence of alteration of pollucite. Wide veins of coarse-grained lepidolite + quartz ± spodumene cut the pollucite, followed by thin veins of fine-grained muscovite and spodumene; both types of veins are overgrown by granular aggregates of (K,Rb)-feldspar and albite. End-member CsAlSi2O6 occurs with these feldspars. The CRK = 100(Cs+Rb+K)/Σcations of the latest pollucite ranges from 97.2-99.9 with Si/Al near 2.0. A mechanism of dissolution and reprecipitation is suggested for its origin, which could remarkably increase its Cs,Al content relative to its Na,Si-rich precursor. Later events (at less than 150-100 °C) caused only analcimization and replacement of pollucite by clay minerals.