Synthesis and crystallogenesis of ferric smectite by evolution of Si-Fe coprecipitates in oxidizing conditions

Abstract

A B S T R A C T : Silico-ferric coprecipitates, with chemical formula S i4Fe23 + O.. nHzO, were aged in suspension at 75 ~ 100 ~ and 150~ and the structural evolution of solids with time studied by XRD, TEM, and IR, Mfssbauer and EXAFS spectroscopy. The initial Si-Fe coprecipitate was found to be amorphous but showed local order similar to that of a smectite layer. At 75~ only a weak structural evolution of the silico-ferric product towards a smectite-like structure was observed. Experiments performed at 100 ~ and 150~ led to synthesis of a ferric smectite with 9 e 3+ C structural formula Sl4F j.s30~0(OH)z %.26. During syntheses a highly soluble silico-ferric complex appeared; the Si/Fe atomic ratio of this complex was 3, and the apparent concentration of Fe 3+ in solution reached 27 mM/l. These syntheses prove that the crystallization of a dioctahedral smectite, containing only Fe 3+ atoms in the octahedral sheet, is possible under strictly oxidizing conditions. However, crystal growth of a ferric smectite under these conditions is slow and only syntheses carried out at sufficiently high temperatures give convincing results.

Formation of Nontronite from Oxidative Dissolution of Pyrite Disseminated in Precambrian Felsic Metavolcanics of the Southern Iberian Massif (Spain)

“…The above genetic interpretation is based not only on the field, mineralogical and geochemical data presented here, but it is also supported by the experimental results of Decarreau et al (1987) and Mizutani et al (1991) who synthesized nontronite from silica gels and iron sulfate under oxidizing conditions.…”

Biogenic Nontronite from Marine White Smoker Chimneys

“…When a spontaneous oxidation, in contact with air, was effected, the ferrous smectite nuclei transformed rapidly into a ferric, nontronite-like smectite. In a later study, Decarreau et al (1987) showed that the crystallization of a di-octahedral smectite, containing only Fe 3+ in the octahedral sheet is also possible under strictly oxidizing conditions. Crystal growth, however, under these conditions is slow and goes to completion only at sufficiently high temperatures.…”

“…Crystal growth, however, under these conditions is slow and goes to completion only at sufficiently high temperatures. Thus, the study of natural occurrences (particularly in the Red Sea) as well as synthesis experiments suggest that hydrothermal nontronite is likely formed in initially reducing conditions, with Fe principally in the ferrous state, and then oxidizing conditions (Decarreau et al 1987).…”

Clay Mineralogy and Occurrence of Ferrian Smectites Between Serpentinite Saprolites and Basalts in Biga Peninsula, Northwest Turkey

“…However, in well-drained environments, Fe-and Al-montmorillonites would be stable long before the composition of pore waters approached the stability field of saponite; such an occurrence is rare in nature (Wildman et al, 1968b). Decarreau et al (1987) studied synthesis and crystallogenesis of ferric smectite in oxidizing conditions and proved that the crystallization of a dioctahedral smectite, containing only Fe 3+ atoms in the octahedral sheet, is possible under strictly oxidizing conditions. They also found that crystal growth of a ferric smectite is slow.…”

“…During weathering, soil mica is transformed first to a vermiculite then to a beidellite, preserving the tetrahedral character of the charge (Badraoui et al 1987;Badraoui and Bloom, 1990). Decarreau et al (1987) synthesized a pure dioctahedral smectite from silico-ferric co-precipitates (Si4Fe3+l.83010(OH)2Ca0.26), at temperatures of 100-150~ Brigatti (1983) investigated chemical and crystallographic properties of natural dioctahedral Fe-rich smectites formed by weathering of basaltic rocks in the Bereci-Euganei, northern Italy. She found that the b-dimension could be correlated both to Fe3+,~ ratio (5~ = octahedral occupancy) and to dehydroxylation peak temperatures in the range 400-600~ total octahedral occupancy is 52 for dioctahedral smectites and in the smectite group there is not a continuous isomorphous series.…”