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Investigation of fluids in macrocrystalline and microcrystalline quartz in agate using Thermogravimetry-Mass-Spectrometry

Richter-Feig, J.; Möckel, R.; Götze, J.; Heide, G.


Gaseous and liquid fluids in agates (banded chalcedony – SiO2) of different localities were investigated systematically by thermogravimetry-mass-spectrometry in a temperature range between 25°C and 1450°C for the first time. Chalcedony and macrocrystalline quartz from twelve agate samples have been investigated: Germany (Schlottwitz, St. Egidien, Chemnitz and Zwickau), Brazil (Rio Grande do Sul), Scotland (Ayrshire) and the USA (Montana). They originate from mafic and felsic volcanic rocks as well as hydrothermal and sedimentary environments.
The results were evaluated concerning compounds of hydrogen with fluorine, chlorine, nitrogen, carbon and sulfur. Additionally, oxygen compounds were recognized with hydrogen, fluorine, nitrogen, sulfur and carbon. The nature of the compounds was identified based on their mass-charge-ratio and the intensity ratios of the associated fragments. Due to interferences of different compounds with the same mass-charge-ratio, only H2O, HF, NO, S, SO, CO3 as well as several hydrocarbon compounds (for example CO32- or CO) could be properly identified. The main degassing temperatures were detected around 500 °C and 1000 °C. Generally, a difference between quartz and chalcedony concerning the composition of their fluids could not be found. The results indicate a silica source for the agate formation from aqueous solutions, but also the possible role of fluorine compounds. Additionally, CO2 and other fluids were involved in the alteration of volcanic rocks and the mobilization and transport of SiO2.

Keywords: agate; quartz; fluids; thermogravimetry-mass-spectrometry; chalcedony