MR44A-01
Birch's Law and Modeling Hydration in the Earth's Interior
Anderson (1969) showed that Birch's Law (1947); "Seismic sound speeds can be plotted as a simple function of density and that curve spreads out according to atomic weight", can be extended to show that bulk modulus has a near linear relationship with reduced density (density divided by mean atomic weight). This potentially useful relationship, however does not apply if hydrogen is included in the mean atomic weight. Hydrogen, however, does not substitute in a normal cation site in either hydrous or nominally anhydrous minerals, but rather finds a position between adjacent oxygen atoms limiting its repulsive potential in the structure. It is therefore reasonable to exclude H from the mean atomic weight calculation. We have found that there is useful relationship that predicts bulk modulus for anhydrous, hydrous and partially hydrated nominally anhydrous minerals. KT = 2747 ( ρ /M) - 308 (GPa) R = 98.5 % where ρ is density, and M is mean atomic weight exclusive of hydrogen. Whereas this requires some knowledge of volumes and mechanisms of hydration to apply to nominally anhydrous minerals that are possible hydration sites in the mantle, such data are forthcoming from laboratory mineral physics.
MR44A-02
Correlation between hydroxyl stretching frequency and O-H...O distance from Raman spectroscopy, crystallography and chemical analysis
Correlations between hydroxyl stretching frequency and O-H...O distance have been widely used in research of
hydroxyl-bearing materials. There have been only two systematic studies to establish such correlations in the
past: one by Nakamoto et al (1955) based on a limited number of relatively simple chemicals and the other by
Libowitzky (1999) from an extended list of minerals. However, spectroscopic and crystallographic data used to
establish these two correlations were mostly from different specimens, which are likely to have different chemical
compositions, especially for natural samples. Difference in compositions may results in variation of bonding
environments and hydroxyl vibration frequencies, and consequently affects their correlations. The two previous
studies also provided very limited data to constrain the correlation for the strong hydrogen bond region where O-
H...O distances are less than 2.6 Å. Furthermore, the two reported correlations were based on infrared and
published crystallographic data at the time. No correlation has ever been established from Raman spectroscopy,
which has become a powerful and widely available technique for identification and characterization of various
materials.
The current study provides the first correlation constructed using the hydroxyl stretching frequency data measured
from Raman spectroscopy. A highly selected set of over 40 minerals from the large collection of natural samples
of the RRUFF project (http:rruff.info) are used for this study. These selected minerals all have simple and well-
established hydrogen positions, and cover the region from the very weak to very strong hydrogen bonding. All
specimens are fully characterized for their Raman spectroscopy, chemistry, as well as crystal structure
particularly for hydrogen positions if existing structural data do not provide sufficient constraint on hydroxyl species
for the specific chemical composition, or such data do not exist. Special efforts are given in this study to
constrain the correlation in the region where hydrogen bonding characteristics changes from week to strong (O-
H...O distance ~ 2.7 Å) and in the region where hydrogen bonding is very strong (O-H...O distance <
2.6 Å).
References
Nakamoto K, Margoshes M, Rundle RE (1955) Stretching Frequencies as a Function of
Distances in Hydrogen Bonds, J Am Chem Soc 77, 6480
Libowitzky E (1999) Correlation of O-H Stretching Frequencies and O-H...O Hydrogen Bond
Lengths in Minerals, Monatshefte für Chemie 130, 1047-1059.
MR44A-03
Syntheses, X-ray Diffraction and Raman Spectroscopic Characterization of Clinopyroxenes With 6-coordinated Si in the Na(Mg0.5Si0.5)Si2O6- CaMgSi2O6 join
Clinopyroxene capable of accomodating 6-coordinated silicon is of fundamental importance towards our understanding of the crystal chemistry of high-pressure minerals in general and chain silicates in particular, both as a reservoir of Si, and because of the effect on the geophysical properties of the mantle. The present study was initiated to understand how the introduction of the Na(Mg0.5Si0.5)Si2O6 component (NaPx) into diopside (CaMgSi2O6) influences the P2/n-C2/c structural stability and to systematically characterize the crystal chemical effects of [6]Si-substitution in clinopyroxenes. Six clinopyroxenes with different compositions within the NaPx-CaMgSi2O6 join were synthesized with a 1000t-multi anvil device at P-T conditions of 15 GPa and 1500 or 1600 °C:
| Run # | Chemical composition | Symmetry |
| JKB2002-2: | Na(Mg0.5Si0.5)Si2O6 | P2/n |
| JKB2005-18: | (Ca0.10Na0.88Mg0.01)(Mg0.56Si0.44)Si2O6 | P2/n |
| JKB2006-6: | (Ca0.21Na0.78Mg0.01)(Mg0.64Si0.37)Si2O6 | P2/n |
| JKB2004-2: | (Ca0.36Na0.56Mg0.08)(Mg0.73Si0.27)Si2O6 | C2/c |
| JKB2005-15: | (Ca0.65Na0.30Mg0.05)(Mg0.90Si0.13)Si2O6 | C2/c |
| JKB2005-11: | (Ca0.86Na0.08Mg0.06)(Mg0.98Si0.03)Si2O6 | C2/c |
MR44A-04
Interrelation Between Elastic Properties Determined by Brillouin Spectroscopy and Thermal Transport Properties of Garnets
Both thermal diffusivity and elastic constants are mineral properties of major relevance to understand the dynamics of the solid earth. It is expected that a relation between sound wave velocities, elastic constants, and thermal transport properties exist. The goal of this study is to improve our understanding of the correlation between elastic properties, thermal transport properties and the mean free path lengths of phonons. We are determining the entire set of elastic constants of different synthetic and natural garnets, for instance the solid solution series Y3Al5O12-Yb3Al5O12, with the recently installed Brillouin scattering system at the GeoForschungsZentrum Potsdam. This system consists of a Nd:YVO4 solid state Laser with a wavelength of 532 nm as light source and a Sandercock-type tandem multipass Fabry-Perot interferometer equipped with a photomultiplier tube for signal detection as spectrometer. All measurements were performed in a 90 degree symmetric forward scattering geometry. An anisotropy in shear sound wave velocities of about 3 % and an anisotropy in compressional sound wave velocity of about 1 % is observed for the Yb-rich garnets, which is in agreement with literature data for Y3Al5O12. We reported the thermal diffusivity of the same garnet single crystals in a previous study using a transient method[1]. Knowledge of both the elastic constants and the thermal diffusivities allows the calculation of the mean free path lengths of phonons. [1] Schilling, F. R. (1999): Eur. J. Mineral. 11, 1115-1124
MR44A-05
Spectrometric and Crystal Chemical Features of Mexican Opals
This paper presents the spectrometric and crystal chemical study of the precious Mexican 'fire' opals. The colour and crystal chemistry of these volcanic opals were investigated with optic and Raman spectrometry, scanning electron microscopy (SEM), and atomic force microscopy (AFM), as well as X-ray diffraction (XRD). The principal characteristic of precious (noble) opals is their play-of-color that is produced by diffraction of white light from three-dimensionally stacked layers of microscopic spheres of silica. Most play-of-color natural opals are opal A (e.g. Australian sedimentary opals) that is amorphous in XRD. Less studied Mexican volcanic opals, with or without play-of-color are by contrast opal CT (for Cristobalite-Tridymite) - that is the XRD shows diffraction peaks for highly disordered Cristobalite, with abundant Tridymite stacking. Two types of spectra were obtained from colours diffracted by precious Mexican 'fire' opals by the Hyperfine Analytical Network spectrometer: spectra due to a mixture of diffracted colours, and the spectra of pure diffracted colours. SEM investigations proved that in general the building blocks of these CT opals are roughly spherical silica particles about 10-40 nm in diameter, and are best seen in AFM. Mexican play-of-color opals shows a higher degree of organization, in which the nanoparticles form pseudospheres of appropriate size for diffraction of visible light (about 200 nm) in a matrix of less acid soluble (probable better crystallized) particles. The wavelenght of diffracted colours were studied as a function of the diameter of these pseudospheres of silica that form mexican opal. There is a continuum of nanostructures between opals with and without play-of color. The more organized the structure and more round the spheres are, the more noticeable the diffraction color appear. To characterize the spectra of precious opal, their spectra must be accurately recorded and their colorimetric parameters calculated. It is then possible to obtain a precise objective description of the diffracted colours produced by natural and synthetic opals. Raman spectrometry a non-destructive technique well adapted to the study of natural and synthetic gem materials also shows different crystal chemical features of Mexican opals.
MR44A-06
The 10 Å phase at high pressure by first principles calculations and implications for the petrology of subduction zones
The structure and the physical properties of the 10 Å phase, Mg3Si4O10(OH)2 · nH2O, with n=0, n=1 and n=2 is explored over a wide range of pressure with density functional theory in the local density (LDA) and generalized gradient approximations (GGA). The equation of state depends strongly on the amount of water. All the investigated structures present a layer offset with respect to the ideal phlogopite like stacking along the x direction that influences the beta angle: tetrahedral rings and water molecules are not exactly aligned along the c nor the c* directions in the n=1 and n=2 structures. Water is bound to the tetrahedral sheets and is located off the midplane of the interlayer. The water dipole vector lies almost parallel to the tetrahedral sheets at zero pressure; the angle it forms with the ab plane increases with pressure in both n=1 (up to 34°) and n=2 (up to 28°) structures. Water molecules act both as acceptor and donor oxygen groups for hydrogen bonds. Water oxygens accept hydrogen from the intra-layer hydroxyls, leading to H-bonds at the boundary between the weak and the strong limit, in agreement with the relatively high Raman frequency at 3620 cm-1 assigned to this vibrational mode. Water molecules donate hydrogen to both upper and lower basal oxygens. Interactions between water molecules and basal oxygens are complicated by the occurrence of polifurcated, mainly asymmetric, bent hydrogen bonds. Nonetheless, within the stability field of the 10 Å phase, LDA results yield distances between basal oxygens and water hydrogen that fall between the weak and strong limit in agreement with Raman signals at 3672 and 3593 cm-1. GGA simulations suggest even stronger interactions between water molecules and basal oxygens.
MR44A-07
Phase stability and shear softening in CaSiO3 perovskite at high pressure
We predict the phase diagram of CaSiO3 perovskite, finding the tetragonal I4/mcm structure transforming to cubic Pm3̆m with increasing temperature. The transition temperature is 1150 K at 0 GPa, and 2450 K at 140 GPa. The c/a ratio of the tetragonal structure is 1.018 at 100 GPa and increases on compression, as does the static enthalpy difference between tetragonal and cubic structures. The elastic constants of the tetragonal phase at static conditions differ substantially from those of the cubic phase with the Voigt-Reuss-Hill shear modulus 29 % less at 100 GPa. Computations are based on density functional theory in the local density and generalized gradient approximations. The phase diagram and high temperature elastic constants are computed using a mean field theory with parameters of the Landau potential determined via structurally constrained density functional theory calculations. We present a simple scheme for systematically searching for the ground state over all perovskite structures derivable from octahedral rotations within the context of symmetry- preserving relaxation, which confirms tetragonal I4/mcm as the ground state in density functional theory. We argue that the experimental x-ray diffraction pattern can be explained by the I4/mcm phase by considering the development of preferred orientation under uniaxial compression.
MR44A-08
QUANTITAIVE IR SPECTROSCOPY WITH UNPOLARIZED LIGHT
A theory on light propagation in weakly absorbing anisotropic minerals is presented by systematic measurements of the infrared absorbance spectra of calcite, olivine and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the maximum principal linear polarized absorbance is smaller than 0.3 or the ratio of maximum and minimum absorbance is close to unity than: (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are linearly proportional to thickness regardless of the direction of the incident light; (2) the measured angular variation of polarized light absorption as a function of maximum and minimum values is indistinguishable from the theoretical predictions [A(θ) = Amaxcos2(θ)+Aminsin2(θ)] within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the maximum and minimum polarized absorbance; (4) the average unpolarized absorbance of randomly oriented grains is exactly one third of the total absorbance, which is the sum of the principal polarized absorbances. Therefore, calibrations relating total absorbance to water concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be used with unpolarized light on a population of randomly oriented sections, provided that enough measurements can be obtained to constrain the average. We show that ten such measurements are sufficient to achieve a petrologically useful accuracy. This method allows the water concentrations in nominally anhydrous minerals from high-pressure experimental runs and fine-grained mantle xenoliths to be measured, for which the preparation of oriented samples of such specimens is usually not feasible.