The Effect of the Starting Mineralogical Mixture on the Nature of Fe-Serpentines Obtained during Hydrothermal Synthesis
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THE EFFECT OF THE STARTING MINERALOGICAL MIXTURE ON THE NATURE OF Fe-SERPENTINES OBTAINED DURING HYDROTHERMAL SYNTHESIS AT 90°C ISABELLA PIGNATELLI1 *
, RÉGINE MOSSER-RUCK1, ENRICO MUGNAIOLI2, JÉRÔME STERPENICH1, 2 AND MAURO GEMMI
1
Georessources UMR 7359 CNRS-UL, Université de Lorraine, BP 70239, 54506 Vandœuvre-lès-Nancy, Cedex, France Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy
2
Abstract—The formation conditions and stability fields of Fe-serpentines are still poorly understood in both terrestrial (natural or anthropic) and extraterrestrial environments. Knowledge of the effects of physical-chemical parameters on compositional and structural features of Fe-serpentines is lacking, and only a few thermodynamic parameters of these minerals are available in the literature. To fill these gaps, the synthesis of these minerals, while controlling all the physicochemical experimental parameters, was undertaken. Two hydrothermal syntheses were carried out at 90°C to investigate the effect of two different starting mineralogical mixtures on the nature of Fe-serpentines. The run products were identified by several analytical techniques (powder X-ray diffraction, transmission, and scanning electron microscopy). Berthierine crystals were synthesized from a starting mixture of kaolinite and metal iron. The berthierines synthesized show different morphologies and iron contents (~3–38 at.%). From a starting mineralogical mixture composed of quartz and metal iron, cronstedtite crystallizes. Most of the crystals are 1M polytypes. Magnetite is always associated with both berthierine and cronstedtite. Lepidocrocite was observed only in the experiment with kaolinite. These experimental results demonstrated that Fe enrichment in serpentines depends on the silicate precursor (kaolinite or quartz) of the starting mixture. The results are also in agreement with the geochemical equilibrium predicted by thermodynamic modeling, i.e. the formation of berthierine and cronstedtite in association with magnetite at the expense of metal iron and silicates. Keywords—Berthierine . Cronstedtite . Fe-serpentines . Hydrothermal synthesis INTRODUCTION Berthierine, cronstedtite, greenalite, and odinite are Febearing serpentines, phyllosilicate minerals which consist of a tetrahedral and an octahedral sheet (T–O or 1:1 layer). Their general formula is M3T2O5(OH)4, where T are the tetrahedral sites generally occupied by Si4+ and Al3+ and M are divalent or trivalent cations in octahedral coordination, such as Fe3+, Fe2+, Al3+, Mg2+. The octahedral sheet contains three M sites but the presence of cations with different oxidation states or valences implies an intermediate occupancy between di- and trioctahedral for most of them. Cronstedtite (Fe2+3–x Fe3+x)(Si2–x Fe3+x)O5(OH)4 is an exception because: (1) there are no vacant sites in the octahedral sheet; and (2) it contains tetrahedrally coordinated Fe3+ (Bailey 1969; Geiger et al. 1983; Smrčok et al. 1994; Hybler et al. 2000,
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