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Abstract

The precipitation of Mg-phyllosilicates is a key process in regulating the (bio)geochemical cycling and budgets of major elements, such as magnesium (Mg) and silicon (Si), in marine and terrestrial aquatic and sedimentary environments. This study provides experimental data on Mg and Si isotope fractionation between laboratory grown sepiolite-type precipitates and fluid at 25 °C. The solids were characterized by Fourier-transform infrared spectroscopy and transmission electron microscopy with energy-dispersive X-ray spectroscopy to determine the mineralogy, chemistry and crystallinity of the precipitates prior to δ26Mg and δ30Si analyses via multi-collector inductively plasma mass spectrometry of the bulk solids and fluids at 21 and 237 days of reaction. The precipitates are poorly crystallized, but reveal a distinct Mg3OH band, a lath-like crystal shape, a Si/(Si + Mg + Al) ratio of ∼0.61 and nanosized domains with an average lattice thickness of 12.4 Å, all of which are characteristic of sepiolite.
Aluminous sepiolite gels precipitated rapidly from the fluids (within 21 days) that contained initial aqueous Si:Mg molar ratios of 1:55 and 1:110 (with [Si]aq-start = ∼1 mmol·L−1), and the apparent Mg and Si isotope fractionation factors
and
were determined to be +0.46 ± 0.03 ‰ and −3.54 ± 0.40 ‰, respectively. At chemical steady state conditions (after 237 days), the experiments yielded chemically more ideal and partly crystallized sepiolite, with
= +0.69 ± 0.03 ‰ and
= −3.54 ± 0.07 ‰. Our results confirm that mineral-fluid fractionation of Mg isotopes depends mainly on the chemistry and crystallinity of the precipitated sepiolite precursors, whereas Si isotope fractionation seems to be less susceptible to change during sepiolite maturation. We conclude that the (nano)structure and crystal chemistry of authigenic Mg-phyllosilicates as well as the physicochemical conditions of the ambient natural fluid have to be carefully assessed for an accurate interpretation of the Mg and Si isotopic signatures recorded in sedimentary clays.