Nepheline-kalsilite microperthites in ejecta from the Alban Hills (Italy)

Abstract

Microprobe analyses carried out on exsolved nepheline and kalsilite phases of microperthites occurring in three ejecta from the Alban Hills (Roman comagmatic region) have shown: (1) a practically continuous compositional variation from Ks to Ne (bulk analysis of microperthites is lacking); (2) two areas of chemical variation according to the Ks/Ne ratio of the bulk analysis of microperthites in its relationship with the solvus of the Ks-Ne system; (3) a larger variation in the chemical compositions of the exsolved phases with regard to the above-mentioned ratio of the microperthite bulk analysis. The lattice parameters of the two phases have been determined, and the Ks a0 value indicates a d-kalsilite for all specimens. With reference of the high part of the Ks-Ne system, the anomalous case may be attributed to the formation of zoned crystals with large compositional variation in a non-equilibrium cooling process. From these crystals the exsolution process may give rise to Ne and Ks domains showing chemical compositions different from those expected.

Keywords

• nepheline-kalsilite microperthites
• Ne-Ks microprobe analyses
• d-kalsilite
• ejecta
• Alban Hills
• Italy