Showing papers on "Tridymite published in 1995"

Application of Fourier Transform Infrared Spectroscopy to Silica Diagenesis: The Opal-A to Opal-Ct Transformation

Abstract: An important goal in silica diagenesis research is to understand the kinetics of opal transformation from noncrystalline opal-A to the disordered silica polymorph opal-CT. Because the conventional technique for monitoring the transformation, powder X-ray diffraction (XRD), is applicable only to phases with long-range order, we used Fourier transform infrared spectroscopy (FTIR) to monitor the transformation. We applied this technique, combined with XRD and TEM, to experimental run products and natural opals from the Monterey Formation and from siliceous deposits in the western Pacific Ocean. Using a ratio of two infrared absorption intensities ( = I472 cm-1/I500 cm-1), the relative prop rtions of opal-A and opal-CT can be determined, The progress of the transformation is marked by changes in slope of vs. depth or time when a sufficient stratigraphic profile is available. There are three stages in the opal-A to opal-CT reaction: (1) opal.A dissolution; (2) opal. CT precipitation, whose end point is marked by completion of opal-A dissolution; and (3) opal-CT ordering, during which tridymite stacking is eliminated in favor of cristobalite stacking.

Cristobalite- and tridymite-bearing clasts in Parnallee (LL3) and Farmington (L5)

Abstract: — A set of cristobalite- and tridymite-rich igneous clasts (CB1 to CB8) have been found in Parnallee (LL3.6). They consist of clinoenstatite, minor feldspathic mesostasis and cristobalite veined by endiopsideaugite. The largest clast, CB8, is 1.6 cm in diameter and contains veined tridymite and cristobalite, clinoenstatite (zoned to ferroaugite and pyroxferroite Fs75.6Wo20.0) and plagioclase. Compared to bulk ordinary chondrites (OC), the bulk clasts are depleted in Al (0.02–0.8× OC), Na and K and enriched in Si (1.6–2.0× OC) and Ca (1.3–4.5× OC). Bulk CB8 has LREE > HREE (La/Lu = 1.6) with a positive Eu anomaly (Eu/Eu* = 2.4). Textural observations suggest that the clasts cooled rapidly (24–420 °C/h) above 1200 °C. Clasts CB1—CB8 contain the isotopically heaviest O yet found in ordinary chondrites (up to δ17O = +8.7%o, δ18O = +11.6%o). Enrichment in the heavy isotopes of O is dependent on the proportion of cristobalite (or tridymite) in the clasts. A regression line CRIL (Cristobalite Line), with slope 0.77, is defined by the isotopic compositions of CB1—CB8, the Farmington clast and ordinary chondrite chondrules. An 16O-poor gas reservoir, whose composition must lie at some point along the extension of CRIL, has undergone varying degrees of isotopic exchange with most ordinary chondrite material. Silica polymorphs have undergone the greatest degree of exchange because of their open, framework structures. Silicon in CB1—CB8 has normal isotopic ratios. A model is proposed that involves differentiation of H-group material through extraction of volatile elements in a vapour phase, loss of an Fe-Ni-S melt and metastable crystallisation (60–70%) of olivine. The calculated residual liquid is silica-oversaturated and its subsequent predicted crystallisation sequence resembles that preserved in CB1—CB8. This model may require two stages of heating, the second one prior to cristobalite crystallisation (if the silica polymorph crystallises within its predicted stability field of > 1500 °C). Isotopic exchange took place either when CB1—CB8 were ejected from their parent body due to impact or near the surface of the parent body, perhaps in an ejecta blanket setting. The latter option is preferred because it is more consistent with our igneous model.

Energy-filtered electron diffraction study of vitreous and amorphized silicas

Abstract: Radial distribution functions deduced from energy-filtered electron diffraction data have been obtained for vitreous silica, neutron-amorphized quartz, ion-amorphized quartz and electron-amorphized quartz, cristobalite and tridymite. Measurable differences in short-range and medium-range structure exist among these six forms of metamict silica.

Thermally induced phase transitions in tridymite: an infrared spectroscopy study

Abstract: The temperature dependence of the infrared active modes of meteoritic and synthetic tridymite have been investigated between 23 K and 1073 K in IR absorption and IR emission experiments. At room temperature both tridymite samples consist of a mixture of low temperature forms, in different proportions, due to the grinding. The sequence of phase transitions in Steinbach tridymite deduced from the IR data agrees well with recent X-ray and calorimetry studies using identical samples (Cellai et al. 1994). The previously suspected structural phase transition P6322⇔P63/mmc is confirmed by the disappearance of the 470 cm-1 mode and a temperature anomaly of the spectral shift of the 790 cm-1 mode. Changes in the infrared spectra of synthetic tridymite give a different sequence of phase transitions from those of the meteoritic sample, consistent with the structural phase transitions observed in a 29Si MAS NMR investigation using the same sample (Xiao et al. 1993).

Investigations of MX-1 Tridymite by 29Si MAS NMR- Modulated Structures and Structural Phase Transitions

Abstract: MX-1 tridymite is one of the room-temperature polymorphs of SiO2 tridymite and has an underlying monoclinic structure (Cc) with incommensurate modulations along a* and c* (Hoffmann et al. 1983; Lons and Hoffmann 1987). With increasing temperature up to 500° C, MX-1 is reported to experience at least five structural phase transitions. However, its structures and the relationships to other tridymite polymorphs are unclear. We present here a 29Si MAS NMR study of the room-temperature incommensurate structure of MX-1 and its structural phase transitions up to 540° C. Our results suggest that at room temperature, all the Si sites in MX-1 tridymite are in positions with similiar ∠Si-O-Si of ∼150° and are consistent with the presence of two incommensurate modulations proposed by Hoffmann et al. (1983). Simulations of the spectra yield modulation amplitudes of 1.33 and 0.87 ppm, corresponding to 0.009 and 0.006 A for Si-Si. The maximum atomic displacements along a and b due to the modulations appear to be ∼0.01–0.02 A. The structural phase transitions of MX-1 are significantly different from those of MC tridymite below 220° C. Our high temperature results confirm that MX-1 tridymite transforms to the H5 phase at about 65° C. The most important transition occurs near 110° C, where the H5 phase transforms to a phase yielding a single, narrow NMR peak, indicating the disappearance of the superstructure and possibly the onset of the dynamic averaging. The NMR lineshapes of H5 are consistent with the metrically orthorhombic unit cell and commensurate superstructure of 2a, 2b and 10c proposed by Graetsch and Florke (1991). The phase present above 110° C is probably similar to the OC phase, but has a mean ∠Si-O-Si of ∼152.0° at 113° C, 152.9° at 185° C and 154.1° at 500° C. The transitions at ∼160 and 220° C for MX-1 are subtle and probably due to impurity MC. Analysis of the modulations in the OS phase of MC tridymite indicates that their amplitudes are of the order of 0.02 A, significantly less than the value 0.3 A proposed by Nukui et al. (1979).

Disorder and successive structure transitions in the tridymite forms of SiO2

Abstract: Disorder models of oxygen positions in P63/ mmc, C2221 and P212121 tridymites were given in applying geometrical and lattice dynamical calculations. Sixmembered rings of rigid SiO4 units are all collapsed in these forms; with silicon atoms fixed, SiO4 units can take six different orientations in forming tridymite frameworks in both the P63/mmc and C2221 forms, and three orientations in the P212121 form. Atomic distances and angles obtained from the distance least-squares method are about equal for the three forms: 〈Si-O〉 (mean Si-O) = 1.611 A, 〈O-O〉 = 2.629 A, and 〈Si-O-Si〉 = 147°. Domain formation models are given for the three forms. The tridymite framework structures may possibly undergo lattice vibrations with low frequencies in two kinds of pair-wise rotational modes of SiO4 units joined by the apical oxygen atoms, at the Γ-point: one is around 〈100〉 (or 〈210〉 for the hexagonal case), and the other is around 〈010〉. As temperature approaches the hexagonal-orthorhombic transition from below, the rotational mode around 〈100〉 remarkably softens at the Γ-point. The behavior of the atoms at the hexagonal-orthorhombic transition is explained in terms of a coupled softening of the two rotational modes of neighboring local domains in different orientations.

Simulation of oxygen vacancies at the Si-SiO2 interface

Abstract: The Si(001)/SiO2 (tridymite) interface has been simulated using a Monte-Carlo method It has been shown that in this way reasonable values of angles and interatomic distances are obtained The oxygen defect formation energy dependence with different vacancy sites has been studied Because of coulombic interactions, the formation of vacancies is much easier in the vicinity of the interface

Nuclear magnetic resonance investigations of the structural phase transitions of AlPO4 tridymite

Abstract: 27Al and 31P NMR spectroscopic data are presented for the tridymite polymorph of AlPO4 (AlPO4 −1) through its structural phase transition at about 80 °C. The RT 27Al and 31P spectra of AlPO4−t both contain doublets of broad peaks, indicating two well-separated groups of sites in the RT structure with mean Al-O-P bond angles per tetrahedron of ∼ 147.8°and 153.1°(±1). With increasing temperature, the doublets remain the same up to about 74 °C, where the relative intensities of the two peaks start to change. The peak corresponding to smaller Al-O-P bond angles disappears, and above ∼88 °C the 27Al and 31P spectra contain single symmetrical peaks, corresponding to a mean Al-O-P bond angle of 153.4°. This bond angle increases gradually with increasing temperature to 153.7°at ∼150 °C and remains constant to about 500 °C. 27Al quadrupole echo experiments suggests that the 27Al nuclear quadrupole coupling constant (QCC) is small and decreases with increasing temperature. QCC remains nonzero in the high temperature phase of AlPO4−t, consistent with the previously proposed 3m local symmetry of Al in the high-temperature structure.

Production of silica brick

Abstract: PURPOSE:To obtain a tridymite-based silica brick contg. a small amt. of residual quartz and having improved creep resistance due to a small amt. of glass by adding specified amts. of a soda lime-silica melt and iron oxide to silica stock. CONSTITUTION:When silica stock based on silica is burnt produce a silica brick, the silica stock as principal stock is blended with 0.05-0.25wt.% powdery soda lime-silica melt having <=0.3mm particle size and a chemical compsn. consisting of, by weight, 70-75% SiO2, 5-13% CaO, 10-40% Na2O, 0.5-2% Al2O3 and 0.5-4% MgO and 0.1-1.0wt.% (expressed in terms of Fe2O3) iron oxide made of FeO.Fe2O3, Fe2O, Fe2O3 or Fe2O-Fe2O3 and having <=0.3mm particle size.

Inhibitior for chemical damage of textile fibres.

Abstract: Crystalline sheet silicas or silicates of the general formula ABSixO2x-1.yH2O where A, B = Na, K or H, x = 7 to 30, y = 0 to 30, exhibit in the X-ray diffraction diagram one or more reflections in the range of d values from 3.0 to 4.0 x 10 cm which cannot be attributed to quartz, tridymite or cristobalite and are used as inhibitors of chemical textile fibre damage.