Present day hydrothermal vents are considered as analogues of sites where live could have emerged on primitive earth. Strýtan (Iceland) is a shallow hydrothermal system composed of vertical Si-Mg structures, whose mineral association corresponds to a low crystallinity silica, trioctahedral smectites (Sm) and smaller amounts of carbonates. To better understand mineral formation in such vent systems we have synthesized trioctahedral Sm in the kerolite (Krl)-stevensite (Stv)-saponite (Sap) series using hydrothermal conditions in slightly alkaline solutions from amorphous hydrous Mg-Al silicates. The starting material consisted of coprecipitated gels with different Si/Mg/Al/Na atomic ratios. Gels were aged with water for 60 days at 150, 175, 200 and 225 °C in Teflon lined reactors. The solid samples were characterized by XRD, DTA-TG, FTIR and TEM.
XRD revealed the formation of low crystallinity phyllosilicates. The 001 reflection evolved with increasing T and Al from 9.7, to 12 and 14.5 Å. The patterns contained peaks corresponding to Krl and Stv/Sap. In oriented mounts, solvation with ethylene glycol shifted the 001 peak to 17.4 Å in gels 2 and 3, whereas in Gel 1 it remained at 9.7 Å with a shoulder at 17.2 Å. FTIR bands were also associated to talc-Krl and Stv-Sap. TEM images of Gel 1 at 225°C showed unresolved aggregates of tiny crystals of 10 nm, with turbostratic structures. Gel 3 produced layered crystals containing numerous defects and with spacings of 10-11 Å, typical of trioctahedral Sm. Thus, aging treatment transformed gels into Krl-Sm interstratified minerals and trioctahedral Sm. T increased the conversion rate from 40 to 75% at 150°C to 225°, respectively. Gel 1 produced a 20-80% Stv-Krl interstratified mineral, composed of tiny crystals. Increasing Al content improved the crystallinity and the % of the Sm. Gel 2 consisted of a Sm-Krl interstratified mineral richer in Sm, while Gel 3 is fully transformed into Sap with tetrahedral Al.
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