The alteration of leucite-bearing rocks: a possible mechanism
Giampaolo, Ciriaco; Godano, Raimοndο F.; Sabatino, Bruno Di; Barrese, Eugeniο
European Journal of Mineralogy Volume 9 Number 6 (1997), p. 1277 - 1292
published: Dec 2, 1997
manuscript accepted: May 12, 1997
manuscript received: Feb 13, 1996
ArtNo. ESP147050906016, Price: 29.00 €
Abstract A series of rocks collected from a single lava flow near Lake Bracciano (Rome) shows progressive stages of analcimization. This paper presents a chemical study of these rocks with calculations of gains and losses from the bulk-rock chemical composition based on atomic percent. With increasing alteration, Na increase is counterbalanced by a decrease in K, Ca and Mg; total iron content is constant but ferrous Fe increases. Mineral phases, either as phenocrysts and/or as laths present in the volcanic groundmass, have been analyzed by EMPA. In addition to leucite altered to analcime, feldspars and pyroxenes converge towards close composition with progressive analcimization. Plagioclases reach labradorite-bytownite compositions and a ternary Na-Ca-K member develops. Chemical variations observed either in bulk-rock composition or among minerals suggest that the analcimization process occurs in an open, reducing environment and that a partial rearrangement of alkalis occurs within the rock. Analcimization also appears to be linked to the rapid cooling of the rock and to the formation of zeolites which are subsequently transformed into analcime.