Extreme chemical conditions of crystallisation of Umbrian Melilitolites and wealth of rare, late stage/hydrothermal minerals

• Published in: Open Geosciences
• Main Authors: STOPPA, Francesco, SCHIAZZA, MARIANGELA
• Other Authors: Stoppa, Francesco, Schiazza, Mariangela
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Melilitolite; ultra peralkaline liquid; Zr Ti mineral; zeolite; hydroxylated silicate sulphate carbonate; Umbria Latium Ultra alkaline District; kola peninsula
• Online Access: http://hdl.handle.net/11564/600912; https://doi.org/10.2478/s13533-012-0190-z

Melilitolites of the Umbria Latium Ultra-alkaline District display a complete crystallisation sequence of peculiar, late-stage mineral phases and hydrothermal/cement minerals, analogous to fractionated mineral associations from the Kola Peninsula. This paper summarises 20 years of research which has resulted in the identification of a large number of mineral species, some very rare or completely new and some not yet classified. The progressive increasing alkalinity of the residual liquid allowed the formation of Zr-Ti phases and further delhayelite-macdonaldite mineral crystallisation in the groundmass. The presence of leucite and kalsilite in the igneous assemblage is unusual and gives a kamafugitic nature to the rocks. Passage to non-igneous temperatures (T<600 °C) is marked by the metastable reaction and formation of a rare and complex zeolite association (T<300 °C). Circulation of low-temperature (T<100 °C) K-Ca-Ba-CO2-SO2-fluids led to the precipitation of sulphates and hydrated and/or hydroxylated silicate-sulphate-carbonates. As a whole, this mineral assemblage can be considered typical of ultra-alkaline carbonatitic rocks.