Anomalous behavior at the I2/a to Imab phase transition in SiO2-moganite: An analysis using hard-mode Raman spectroscopy (original) (raw)
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Evidence for an I2/a to Imab phase transition in the silica polymorph moganite at~ 570 K
2001
Rietveld analysis of synchrotron X-ray powder diffraction data for the silica mineral moganite from 100 K to 1354 K has revealed a reversible phase transition from space group I2/a to Imab at approximately 570 K. The thermal expansion behavior of the lattice parameters alters sharply at the transition point, and the monoclinic β angle decreases to 90°. The displacive transition from αto βmoganite is effected by the rotation of apparently rigid tetrahedra about the [010] axis, and the linear temperature dependence of the volume strain and of the non-symmetry-breaking e 11 and e 22 strains indicates that the character of the transition is second-order. The continuous increase in the b axis over the entire temperature range reveals a concomitant rotation of tetrahedra about [100] that does not affect the overall symmetry. In addition, we present a refinement without structural constraints for α-moganite at room temperature using time-of-flight neutron diffraction data.
American Mineralogist, 1996
We performed in-situ Raman spectroscopy and X-ray diffraction experiments at high temperature and ambient pressure to investigate the intrinsic anharmonic properties of {J-Mg2Si04 and the mechanism and kinetics of its back-transformation to forsterite. Hightemperature Raman spectra of {J-Mg2Si04 and its back-transformed products were recorded up to 1200 K. {J-Mg2Si04 persists metastab1y up to 800-900 K, and the Raman frequency shifts with temperature were determined. Between 800 and 1000 K, new peaks are observed at about 670 and 1020 cm-I. Above 1000 K, a direct transformation to forsterite occurs. The peaks that appear between 800 and 1000 K are attributed to a defective spinelloid that forms as an intermediate phase during the back-transformation of {J-Mg2Si04 to forsterite. Similar features are observed in the Raman spectrum of partially transformed ')'-Ni2Si04 heated at 1073 K and ambient pressure for 10 min. These results indicate that a two-step mechanism, possibly martensitic, is operative in the backtransformation of the {J-and ')'-phases to olivine at low to moderate temperatures and for a large overstepping of the equilibrium conditions. The kinetics of the {J-to a-Mg2Si04 back-transformation were monitored between 1023 and 1120 K at ambient pressure using X-ray powder diffraction. For the kinetic data obtained in air, two regimes are evident from an Avrami analysis. The first regime is characterized by an exponent n "" 2 for a low transformed fraction (X < 0.5); the second has n "" 1 for higher transformed fractions. For this second regime, an activation energy of 432 :t 64 kllmo1 is derived for the growth process from the kinetic data. A smaller data set collected in vacuum indicates much slower transformation rates and suggests a significant effect of the O2 or H20 partial pressures on the kinetics. Intrinsic mode anharmonic parameters can be calculated from the Raman frequency shifts with temperature and used to correct vibrational heat capacity models for intrinsic anharmonic effects. These corrections are slightly higher for the {J-phase than for forsterite but the difference is within the experimental error. This indicates that, within the resolution of our experiments, no significant effect of intrinsic anharmonicity on the location and slope of the a-{J phase transition can be predicted.
American Mineralogist, 1996
We performed in-situ Raman spectroscopy and X-ray diffraction experiments at high temperature and ambient pressure to investigate the intrinsic anharmonic properties of {J-Mg2Si04 and the mechanism and kinetics of its back-transformation to forsterite. Hightemperature Raman spectra of {J-Mg2Si04 and its back-transformed products were recorded up to 1200 K. {J-Mg2Si04 persists metastab1y up to 800-900 K, and the Raman frequency shifts with temperature were determined. Between 800 and 1000 K, new peaks are observed at about 670 and 1020 cm-I. Above 1000 K, a direct transformation to forsterite occurs. The peaks that appear between 800 and 1000 K are attributed to a defective spinelloid that forms as an intermediate phase during the back-transformation of {J-Mg2Si04 to forsterite. Similar features are observed in the Raman spectrum of partially transformed ')'-Ni2Si04 heated at 1073 K and ambient pressure for 10 min. These results indicate that a two-step mechanism, possibly martensitic, is operative in the backtransformation of the {J-and ')'-phases to olivine at low to moderate temperatures and for a large overstepping of the equilibrium conditions. The kinetics of the {J-to a-Mg2Si04 back-transformation were monitored between 1023 and 1120 K at ambient pressure using X-ray powder diffraction. For the kinetic data obtained in air, two regimes are evident from an Avrami analysis. The first regime is characterized by an exponent n "" 2 for a low transformed fraction (X < 0.5); the second has n "" 1 for higher transformed fractions. For this second regime, an activation energy of 432 :t 64 kllmo1 is derived for the growth process from the kinetic data. A smaller data set collected in vacuum indicates much slower transformation rates and suggests a significant effect of the O2 or H20 partial pressures on the kinetics. Intrinsic mode anharmonic parameters can be calculated from the Raman frequency shifts with temperature and used to correct vibrational heat capacity models for intrinsic anharmonic effects. These corrections are slightly higher for the {J-phase than for forsterite but the difference is within the experimental error. This indicates that, within the resolution of our experiments, no significant effect of intrinsic anharmonicity on the location and slope of the a-{J phase transition can be predicted.
Kinetic and thermodynamic properties of moganite, a novel silica polymorph
Geochimica et Cosmochimica Acta, 1997
growing body of evidence reveals that much of the silica that crystallizes at the Earth's surface is a finely intergrown mixture of quartz and moganite. To better understand the behaviour of both solid and aqueous silica in these systems, the kinetics and thermodynamic properties for endmember moganite have been determined as a function of temperature from 2.5" to 200°C. Because endmember moganite has yet to be found in nature or synthesized in the laboratory, these properties were determined indirectly by (1) measuring quartz dissolution rates at pH 3.5, (2) measuring the dissolution rates of
Physics and Chemistry of Minerals, 2012
Chalcedony is a spatial arrangement of hydroxylated nanometre-sized a-quartz (SiO 2 ) crystallites that are often found in association with the silica mineral moganite (SiO 2 ). A supplementary Raman band at 501 cm -1 in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica rocks. Its frequency lies at 503 cm -1 in sedimentary chalcedony, representing a 2 cm -1 difference with its position in pure moganite. We present a study of the 503 cm -1 band's behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300°C. Infrared spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good correlation between the disappearance of the 503 cm -1 Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm -1 in chalcedony, assigned to 'free' Si-O vibrations of non-bridging Si-OH that oscillate with a higher natural frequency than bridging Si-O-Si (at 464 cm -1 ). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si-OH-related band is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take into account the contribution from silanole that are present in chalcedony and moganite.
High‐pressure metastable phase transitions in β‐Ge3N4 studied by Raman spectroscopy
Journal of Raman …, 2003
We studied polymorphic transitions occurring in b-Ge 3 N 4 metastably compressed to high pressures (P = 40 GPa) at room temperature. Previous studies under high-pressure-high-temperature conditions have shown that this phase transforms into a newly discovered spinel-structured material (g-Ge 3 N 4 / above 10 GPa and 1200 • C. However, ab initio theoretical studies have indicated that the phenacite-structured low-pressure b-Ge 3 N 4 polymorph should also undergo a series of metastable transformations upon compression at low temperatures. Here we studied these transformations by in situ Raman spectroscopy in a diamond anvil cell. The phase transitions were first predicted to involve a sequence of second-order phase changes driven by soft phonon modes at the Brillouin zone centre (the 0 point: q = 0) resulting in displacive motions of the N atoms, causing the symmetry to descend from P6 3 /m through P6 to P3. The two transitions were predicted to occur at P = 20 and 28 GPa. However, it was also noted that a direct first-order transition between P6 3 /m and P3 phases might occur at P = 23 GPa if order-disorder processes or phonon condensations away from the Brillouin zone centre were considered. Here we present experimental evidence for a phase transition occurring at P = 20 GPa, within b-Ge 3 N 4 that has been metastably compressed at ambient temperature. From the number of Raman modes observed, this transition most likely corresponds to the direct P6 3 /m-P3 transformation, and it is therefore first order in character, but with a small activation energy barrier as ascertained by little or no hysteresis observed upon decompression. However, when all the minor features appearing in the Raman spectrum are accounted for, the number of observed resonances is greater than the number of zone centre modes expected even for the P3 structure. This indicates that the true unit cell is larger than expected from zone-centre mode softening, and that order-disorder processes or phonon instabilities at q 6 = 0 must have occurred during the transition.
Moganite detection in silica rocks using Raman and infrared spectroscopy
European Journal of Mineralogy, 2014
The quantitative determination of moganite in flint and chert plays an important role in the characterisation of these silica rocks and in the study of their genesis and evolution. Both Raman and infrared (IR) spectroscopy promise to be rapid and costeffective tools for such studies. However, the use of vibrational spectra of moganite in silica rocks is hampered by the proximity of specific moganite bands with IR and Raman vibrations bands of non-bridging Si-O in silanol (SiOH) groups of chalcedony, the main coexisting silica phase. This may result in spectral interferences that lead to an overestimation of the moganite concentration. In order to calibrate quantitative moganite detection using IR and Raman spectroscopy, the spectra of chalcedony/moganite mixtures were studied using spectral decomposition. Heat treatment of the samples prior to their analysis is found to reduce the contribution of chalcedony silanol-bands to the measurement of the moganite bands, facilitating in this way the interpretation of the spectra. A new calibration curve is proposed for quantitative moganite detection using Raman spectroscopy. Infrared spectroscopy is also found to be useful for moganite quantification: a molar absorption coefficient of 43 L/molÁcm for the specific moganite-band at 575 cm À1 is derived for the first time. The exact position of the specific IR and Raman moganite-bands is found to depend on whether the mineral occurs intermixed with chalcedony or in pure form. This study opens new prospects for quantitative moganite detection in silica rocks using vibrational spectroscopy.
Physics and Chemistry of Minerals, 1999
A theoretical analysis and computer modelling of quartz gives a picture of the ab phase transition in terms that would appear to be widely applicable to other silicate framework structures. The picture is based upon the fact that the structure of the b-phase can distort to the a form through the rotations of the SiO 4 tetrahedra without necessitating any distortions of the individual tetrahedra. A simple model based upon this premise and augmented by lattice energy calculations of the ordering potential gives a value for the phase transition temperature that is in semi-quantitative agreement with experiment. The reasons for the success of this model are then explored using a full anharmonic lattice dynamics calculation of the phase transition using renormalised phonon theory, highlighting the particular significance of the soft phonon branch compared to all the other phonon branches. The basic picture also explains the variation of the transition temperature with cation concentration in other aluminosilicates, and gives further insight into the validity of the mean-field description in these phase transitions.
Calculation of the Raman Linewidths of Lattice Modes Close to the α-β Transition in Quartz
High Temperature Materials and Processes, 2012
The Raman frequencies of the lattice modes (147 cm−1 and 207 cm−1) are analyzed for the α-β transition in quartz according to a power-law formula with the critical exponent by using the experimental data. The temperature dependence of the Raman frequency is associated with the order parameter (polarization P) for this transition in the quartz crystal.The damping constant of the lattice modes studied here is calculated using the Raman frequencies at various temperatures for the α-β transition in quartz (Tc = 846 K) using the soft mode – hard mode and the energy fluctuation models. Our calculations for the damping constant (bandwidths) give an evidence that the lattice mode of the 147 cm−1 exhibits a soft mode behavior for the α-β transition in quartz.