Synthesis of Nanocrystalline Low Temperature Silica Polymorphs.

Abstract

Synthesis of Nanocrystalline Low Temperature Silica Polymorphs. -The low-temp. α-quartz modification is obtained from the hydrothemal treatment of colloidal SiO2 over a wide condition range (125-220°C, 100 MPa water pressure, 5-90 d). The samples are characterized by powder XRD, SEM, and TEM. The α-quartz prepared at ≤145°C is nanocrystalline and may be interesting for its adsorption-desorption properties combined with inertness with respect to pharmaceutical and biochemical substances. -(SCHAEF*, O.; GHOBARKAR, H.; GARNIER, A.; VAGNER, C.; LINDNER, J. K. N.; HANSS, J.; RELLER, A.; Solid State Sci. 8 (2006) 6, 625-633; MADIREL, CNRS, Univ. Provence, F-13331 Marseille, Fr.; Eng.) -W. Pewestorf 36-199

Discovery of moganite in a lunar meteorite as a trace of H 2 O ice in the Moon’s regolith

“…Most carbonaceous chondrites contain large amounts of water species and hydrous products (for example, serpentine) that originated from alkaline fluids within a pH range of 7.0 to 12.0 in the parent bodies ( 36 ). These conditions are in good agreement with the pH range of 9.5 to 10.5 and temperature range of 363 to 399 K of the fluid activities that precipitate both moganite ( 17 ) and SiO 2 nanoparticles with radii <10 nm ( 31 ) on the sunlit surface ( 33 ). This finding implies that this alkaline water would have been delivered to the PKT via carbonaceous chondrite collisions.…”

“…S1 and S4) should also be present in the gabbroic lithologies of NWA 2727 and the paired lunar meteorites NWA 2977, 3333, and 6950, if they were produced by such terrestrial weathering, but this scenario was not the case. These facts also demonstrate that moganite does not form without brecciation under high consolidation pressure, as indicated in previous works (6)(7)(8)(13)(14)(15)(16)(17)(18)(19). Furthermore, moganite is not observed in other lithic lunar meteorites (NWA 032, 4734, 5000, 5744, 6355, 8599, 8668, and 10649 and Dar al Gani 400), which indicates that its existence is a unique characteristic of the gabbroic-basaltic breccia matrix of NWA 2727.…”

“…S4), not even in the grain rims that would have been most significantly affected by terrestrial weathering. The possibility that the moganite in NWA 2727 was transformed from unhydrated silica minerals (for example, quartz, tridymite, and cristobalite) is ruled out by these observations, even if silica-water interaction occurred during terrestrial weathering, as indicated by previous work ( 17 , 18 ). Only a few hot desert meteorites contain weathering products of vein-filling silica coexisting with hydrous minerals (jarosite and dissolved olivine/pyroxene) ( 20 ).…”

“…Moganite is a metastable phase of monoclinic SiO 2 that belongs to the I2/a space group (6). On Earth, it has been shown that moganite contents of 5 to 100 wt % form through precipitation of nano-to microcrystalline SiO 2 from alkaline fluids only in sedimentary settings (that is, under high consolidation pressure) (6)(7)(8)(13)(14)(15)(16)(17). Thermochemically unstable moganite readily transforms into quartz or dissolves during silica-water interactions at ambient pressure, resulting in moganitefree phases in altered rocks exposed to weathering processes (13)(14)(15)(16)18).…”

“…Moreover, no terrestrial rocks with moganite older than 130 million years (Ma) ago have been found because its thermochemical instability causes the moganite content to decrease with age (6)(7)(8)(13)(14)(15)(16). On the basis of laboratory hydrothermal experiments, moganite has been synthesized via high pressure-induced polycondensation (about >100 MPa) and dehydroxylation of colloidal silicic acid upon changes in pH from 9.5 to 12.0 to 13.0 at 373 to 418 K (17,18). Molecular dynamics simulations of SiO 2 phases (19) have also demonstrated that moganite has an orthorhombic symmetry (space group Ibam) at ambient pressure, whereas monoclinic moganite (I2/a) in a naturally occurring form is stable in the pressure range from 6 to 21 GPa at 300 K. According to these previous investigations, on Earth, some monoclinic moganite has been preserved at concentrations from 5 to 100 wt % in unaltered and consolidated sedimentary rocks (for example, evaporite, chert, and cavities in sedimentary breccia) younger than 130 Ma old (that is, no burial), and it is apparently absent in terrestrially weathered specimens (6)(7)(8)(13)(14)(15)(16)(17)(18).…”