Occurrence and Distribution of Moganite and Opal-CT in Agates from Paleocene/Eocene Tuffs, El Picado (Cuba)

Minerals

Gerardo Orozco¹  Tanja Mohr-Westheide²  Moritz Liesegang³  Klaus Stanek⁴  Jens Götze⁵ 

[1] Departamento de Geología, Universidad de Moa, Avenida Calixto Garcia Iñiguez 15, Moa, Holguín 83330, Cuba;Institute for Evolution and Biodiversity Science, Museum für Naturkunde, Invalidenstraße 43, 10115 Berlin, Germany;Institute for Geological Sciences, FU Berlin, Malteserstraße 74–100, 12249 Berlin, Germany;Institute for Geology, TU Bergakademie Freiberg, Cottastraße 4, 09599 Freiberg, Germany;Institute for Mineralogy, TU Bergakademie Freiberg, Brennhausgasse 14, 09599 Freiberg, Germany;

关键词: agate;    alpha-quartz;    chalcedony;    moganite;    opal-CT;    Raman spectroscopy;   

DOI  :  10.3390/min11050531

来源: DOAJ

【 摘 要 】

Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl-silicate complexes), in the agates confirm their contemporaneous supply with SiO₂ and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO₂ with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A₁ modes) of alpha-quartz at 465 cm⁻¹ and moganite at 502 cm⁻¹, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO₂ under varying physico-chemical conditions and a discontinuous silica supply.

【 授权许可】

Unknown