Pressure–induced crystallization of biogenic hydrous amorphous silica

Abstract

Samples of the diatom Nitzschia cf. frustulum, collected from Lake Yogo, Siga Prefecture, Japan, were cultured in the laboratory. Organic components of the diatom cell were removed by washing with acetone and sodium hypochlorite. The remaining frustules were studied by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX), Fourier-transform infrared (FTIR) spectroscopy, and synchrotron X-ray diffraction. The results showed that the spindle-shaped diatom frustule was composed of hydrous amorphous silica. Pressure-induced phase transformation of the diatom frustule was investigated by in situ Raman spectroscopic analysis. With exposure to 0.3 GPa at 100°C, the Raman band corresponding to quartz occurred at ν = 465 cm −1 . In addition, a characteristic Raman band for moganite was observed at 501 cm −1 . From the integral ratio of Raman bands, the moganite content in the probed area was estimated to be approximately 50 wt%. With increased pressure and temperature, the initial morphology of diatom frustules was totally changed to a characteristic spherical particle with a diameter of about 2 µm. Increasing pressure to 5.7 GPa at 100°C resulted in the appearance of a Raman band assignable to coesite at 538 cm −1 . That is, with compression and heating, hydrous amorphous silica can be readily crystallized into quartz, moganite, and coesite. First-principles calculations revealed that a disiloxane molecule with a trans configuration is twisted 60°with a close approach of a water molecule, which leads to a trans to cis configuration change. It is therefore reasonable to assume that during crystallization of hydrous amorphous silica, an Si-O-Si bridging unit with the cis configuration would survive as a structural defect, and could subsequently crystallize into moganite by maintaining that geometry. This hypothesis is adaptable to the phase transformation from hydrous amorphous silica to coesite as well, because coesite has four-membered rings and is easily formed from hydrous amorphous silica under high pressure and temperature conditions.

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

“…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 ).…”

“…These mineralogical descriptions and laboratory experiments ( 6 – 8 , 13 – 18 ) imply that moganite (37 to 96 wt % with an average of 77 wt %) in NWA 2727 is highly likely to have been precipitated from high-pressure alkaline fluid activity on the Moon, rather than being formed under ambient terrestrial weathering conditions in the hot desert where the meteorites were found. In addition, moganite is exclusively distributed in the breccia matrix of NWA 2727, whereas it is absent from the remaining lithologies and other lunar meteorites.…”

“…Thus, these observations imply that the coesite and stishovite discovered in NWA 2727 were not produced by phase transitions from these silica varieties except for moganite. Solid-state phase transition of moganite to coesite occurs more readily than solid-state phase transitions of other silica varieties such as quartz, tridymite, and cristobalite because of its significantly lower kinetic barrier ( 18 , 19 , 25 ). This low kinetic barrier can be explained by the following two factors: the structural similarity of moganite and coesite and the presence of available water.…”

Stabilization of β-carotene embedded in a silica matrix and study of its physical properties

“…Finally, when the sample was heat-treated at 1000°C for 1 hour, the particles become rounded and spherical with uniform size, as the micrograph of figure 5(c) shows. These morphological characteristics are typical in some silica polymorphs, such as in the like-stishovite structure [38,39]. Figure 6 shows the emission spectra of the βC embedded in a silica matrix as a composite at different calcination temperatures from RT to 1000°C.…”

Temperature–induced phase transition of AlPO₄–moganite studied by in–situ Raman spectroscopy

“…Good-quality SiO 2 -moganite samples are difficult to obtain, and natural samples are often intimate mixtures of moganite, quartz and amorphous silica. Although there are several reports on synthesis of SiO 2 -moganite (Schäf et al, 2006;Kyono et al, 2017), these synthetic samples also contain coexisting quartz. Since many Raman peaks of moganite overlap with those of quartz as a result of their structural similarity (Kingma and Hemley, 1994), it is difficult to study natural SiO 2 -moganite by Raman spectroscopy.…”