The Molecular Species in Cordierite: The Nature of the Fluid Phase During Crystallization

There is much interest in investigating fluid-geomaterial interactions because the fluid phase can strongly affect the nature of many geochemical and petrological processes. A few silicates can trap or occlude various molecules or atomic species like Ar, He, SO2, N2, H2S, H2, CH4 and HCl in micro/nano cavities or channel ways. Cordierite, ideal formula (Mg,Fe)2Al4Si5O18x(H2O,CO2), is such a microporous silicate typically found in metamorphic rocks and in certain igneous rocks. It can be very useful in providing information on the nature of complex fluid phases present during crystallization. Twenty cordierite samples from a wide range of petrologic environments were investigated o determine their molecules. In order to characterize them, a mass spectrometry study was undertaken. The method, DEGAS, requires only small milligram quantities of material, which are heated continuously at a controlled rate from 25 °C to 1400 °C. The results show that H2O and CO2 are the most abundant primary occluded molecules and that their degassing temperatures generally lie between 750 and 1000 °C and 750 and 1200 °C, respectively. H2O contents range from approximately 0.0 wt % to 2.34 wt % and CO2 from 0.0 wt % to 1.97 wt %. Ratios of XCO2/XH2O for samples containing measurable amounts of both molecules range from about 0.2 to 5.6 and there exists an inverse relationship between CO2 and H2O contents in most natural cordierites. Cordierites from pegmatites and middle-grade metamorphic rocks are relatively rich in H2O and poor in CO2, whereas cordierites from the granulite facies have more CO2 than H2O. A number of other occluded molecular or atomic species were identified at minor and/or trace concentration levels. They include 40/36Ar, He, SO2, N2, H2S, H2 and Na and possibly CH4 and HCl. The presence of primary CO could not be ascertained. The concentrations of primary occluded atoms and molecules in natural cordierite can be described approximately as: H2O ~ CO2 >> [Ar ~ He ~ N2 ~ H2S ~ SO2 ~ H2 ~ CH4 and HCl]. The types of molecules and atoms that are incorporated in cordierite and their concentrations depend on three factors: i) The temperature und pressure during crystal growth, ii.) Crystal-chemical and molecular properties and iii.) The composition of the paleo-fluid under which the cordierite crystal crystallized. The DEGAS profiles sometimes show degassing peaks for Ar, H2S, SO2 and possibly a CH3 species, for example, at low temperatures around 150 °C. They are likely related to adsorbed surface species, whose exact physical and chemical state is not clear. Strong degassing “events” are also observed for some species at temperatures between about 300 °C and 700 °C and they are possibly related to very minute solid-phase inclusions such as those associated with pinite.