Composition and structural state of alkali feldspars from ejecta in the Roman potassic province, Italy ; petrological implications.
Della Ventura, Giancarlo; Di Lisa, Giuseppe Antonino; Marcelli, Marco; Mottana, Annibale; Paris, Eleonora
European Journal of Mineralogy Volume 4 Number 3 (1992), p. 411 - 424
published: Jun 11, 1992
manuscript accepted: Jul 17, 1991
manuscript received: Dec 26, 1990
ArtNo. ESP147050403010, Price: 29.00 €
Abstract The alkali feldspars in ejecta of syenitic composition found throughout the pyroclastic deposits of the northern Sabatini volcanic complex (Roman potassic province, central Italy) have been characterized chemically and structurally by means of unit-cell parameters and electron microprobe analyses. Their composition is Or-rich (Or 77 to 95), and is normally distributed around Or 89, with up to 7.8 mol% Sr-feldspar component in some crystals. All are sanidines (degree of order Σt1 < 0.69), and have anomalous cell dimensions (Δa -0.02 to -0.11). The degrees of order show a bimodal distribution: all sanidines (A) of the ejecta in the lower pyroclastic flow of the major centre, the Sacrofano caldera, are ordered (Σt1 = 0.63 - 0.68), whereas sanidines (B) of the ejecta occurring in the minor satellite centres, and most of those in the Sacrofano upper pyroclastic flow are disordered (Σt1 = 0.54 - 0.62). The more ordered A-sanidines are structurally, and to a lesser extent also compositionally compatible with those occurring as phenocrysts in trachytic dykes intersected at various depths in the geothermal wells of the area. Consequently, it is tentatively inferred that all ejecta are dismembered fragments of two types of shallow instrusions which were explosively brought to the surface by the volcanic activity, the sanidine keeping its frozen-in structural state