Hydrothermal genesis and growth of the banded agates from the Allumiere-Tolfa volcanic district (Latium, Italy)

Abstract

In this work, we studied the hydrothermal agates from the Neogene–Quaternary volcanic district of Allumiere-Tolfa, north-west of Rome (Latium, Italy) using a combination of micro-textural, spectroscopic, and geochemical data. The examined sample consists of (1) an outer cristobalite layer deposited during the early stages of growth, (2) a sequence of chalcedonic bands (including i.e., length-fast, zebraic, and minor length-slow chalcedony) with variable moganite content (up to ca. 48 wt%), (3) an inner layer of terminated hyaline quartz crystals. The textures of the various SiO2 phases and their trace element content (Al, Li, B, Ti, Ga, Ge, As), as well as the presence of mineral inclusions (i.e., Fe-oxides and sulfates), is the result of physicochemical fluctuations of SiO2-bearing fluids. Positive correlation between Al and Li, low Al/Li ratio, and low Ti in hyaline quartz points to low-temperature hydrothermal environment. Local enrichment of B and As in chalcedony-rich layers are attributed to pH fluctuations. Analysis of the FT-IR spectra in the principal OH-stretching region (2750–3750 cm−1) shows that the silanol and molecular water signals are directly proportional. Strikingly, combined Raman and FT-IR spectroscopy on the chalcedonic bands reveals an anticorrelation between the moganite content and total water (SiOH + molH2O) signal. The moganite content is compatible with magmatic-hydrothermal sulfate/alkaline fluids at a temperature of 100–200 °C, whereas the boron-rich chalcedony can be favored by neutral/acidic conditions. The final Bambauer quartz growth lamellae testifies diluted SiO2-bearing solutions at lower temperature. These findings suggest a genetic scenario dominated by pH fluctuations in the circulating hydrothermal fluid.

Agates of the Lece Volcanic Complex (Serbia): Mineralogical and Geochemical Characteristics

“…The presence of Bambauer quartz (Figure 4E) in vein agates indicates its formation in hydrothermal fluids characterized by fluctuations of silica concentrations and pH conditions [6,28,[53][54][55][56]. It is supposed that these fluctuations in physicochemical conditions…”

“…The presence of Bambauer quartz (Figure 4E) in vein agates indicates its formation in hydrothermal fluids characterized by fluctuations of silica concentrations and pH conditions [6,28,[53][54][55][56]. It is supposed that these fluctuations in physicochemical conditions of hydrothermal fluids are responsible for the frequent alterations of different silica phases [53] in vein agates, especially in the Mehane and Rasovača agate occurrences.…”

Similarities and differences among selected gemmological varieties of chalcedony: chemistry, mineralogy and microstructure

“…Bands with less intense colouration are composed primarily of granular macrocrystalline quartz. Specifically, the macrocrystals have micrometre-sized lamellae, due to the episodic stages of crystal growth, typical of the so-called 'Bambauer quartz' (Bambauer et al, 1961;Conte et al, 2022) and a feathery aspect at the margins of quartz grains (Adams, 1920;Dong et al, 1995). Granular quartz occurs with different orders of magnitude in size and exhibits undulose extinction (Adams et al, 1984), related to the granules being polycrystalline (Adams et al, 1984), denominated strained quartz (Jóźwiak-Niedźwiedzka et al, 2017).…”

Mineralogy of Agates with Amethyst from the Tevinskoye Deposit (Northern Kamchatka, Russia)

“…The distribution of molecular water in the banded area of agates is not homogenous. The spatial water distribution across the growing sequence (presented by the alternation of moganite-rich and moganite-poor chalcedony layers, and macrocrystalline quartz) of the agate sample from the Allumiere-Tolfa volcanic district (Latium, Italy) was studied by Conte and his colleagues using coupled Raman and FT-IR micro-spectroscopy [49]. In particular, these authors revealed an anticorrelation between the moganite content and water signal.…”

“…The 3585 cm −1 infrared absorption band is occasionally observed in natural and synthetic rock crystals, including amethysts [50]. It was also recognized in the IR spectra of agates from young volcanic hosts, but has not been found in the IR spectra of agates in volcanics aged from 1100 to 89 Ma [44,48,49]. Up to now, the assignment of this band has been quite vague.…”