Micro-, to nano-structural relationships in natural serpentines, derived from cationic substitutions.

Abstract:

The understanding of the crystal chemistry of serpentine minerals (incl. antigorite, lizardite and chrysotile) is fundamental since serpentinization processes concern very large scientific domains: e.g., natural abiotic hydrogen production (Marcaillou et al., 2011), origins of life (Russell et al., 2010), fluid properties and mobility of metals in subduction zones (Kelley and Cottrell, 2009). This study aims at characterizing relations between the micro-, and nano-structures of the most abundant serpentine polytypes in the oceanic crust. Serpentine theoretical formula is Mg₃Si₂O₅(OH)₄ but several natural substitutions are possible and the formula may be written such as: (Mg,Fe²⁺,Fe³⁺,Al)₃(Si,Al,Fe³⁺)₂O₅(OH)₄; showing that Fe and Al may play an important role in the crystallization of serpentines. Preliminary crystal chemistry studies, suggest that, 1) the Al content alone cannot be directly correlated to serpentine polytypes (Andreani et al., 2008), 2) the amounts of tetrahedral iron can be significant in the presence of ferric iron (Marcaillou et al., 2011). Because magnetite is usually associated to serpentine, the Fe-speciation characterization of serpentine is delicate. Here, we provide the study of 33 magnetite-free serpentines containing various amounts of Fe and Al. The samples were characterized by SEM, Raman, XRF, as well as XANES, pre-edge, and EXAFS spectroscopy at the Fe K-edge. XANES experimental data were crosschecked and interpreted thanks to ab initio calculations and EXAFS shell-fitting. Also, preliminary ²⁷Al-RMN data is presented. Results suggest relationships between the type and amount of substitution of trivalent cations in minerals, and the microstructures observed. Chrysotile incorporates less trivalent cations than other varieties, which tends to preserve the so-called misfit between the TO layers, and therefore the tubular structure of the mineral. Lizardites mainly involve Fe/Al Tschermak-type substitutions, while M-site vacancy charge-compensation mechanisms could be favored for antigorite crystals.

References

Andréani et al., 2008, European Journal of Mineralogy, 20, 159-171.

Kelley and Cottrell, 2009, Science, 325, 605-607.

Marcaillou et al., 2011, Earth And Planetary Science Letters, 303, 281-290.

Russell et al., 2010, Geobiology, 8, 355-371.