Ann Graeme-Barber

Metamictization of zircon: Raman spectroscopic study

Raman spectroscopy of radiation-damaged natural zircon samples shows increased line broadening and shifts of phonon frequencies with increasing radiation dose. Stretching and bending frequencies of SiO4 tetrahedra soften dramatically with increasing radiation damage. The frequency shifts can be used to determine the degree of radiation damage. Broad spectral bands related to Si-O stretching vibrations between 900 and 1000 cm-1 were observed in metamict/amorphous zircon. The radiation-dose-independent spectral profiles and the coexistence of this broad background and relative sharp Raman modes in partially damaged samples indicate that these bands are correlated with amorphous domains in zircon. The spectral profiles of metamict zircon suggest that in comparison with silica, the SiO4 tetrahedra are less polymerized in metamict zircon. This study also shows that ZrO2 and SiO2 are not the principal products of metamictization in zircon. No indication of bulk chemical unmixing of zircon into ZrO2 and SiO2 was found in 26 samples with a large variation of radiation damage (maximum dose: 23.5 × 1018 -events g-1 ). Only one sample showed clearly, in all measured sample areas, extra sharp lines at 146, 260, 312, 460 and 642 cm-1 characteristic of tetragonal ZrO2 . The geological (and possibly artificial heating) history of this sample is not known. It is concluded that radiation damage without subsequent high temperature annealing does not cause unmixing of zircon into constituent oxides.

Phonon spectra of alkali feldspars; phase transitions and solid solutions

Abstract The phonon spectra of IR-active vibrations in alkali feldspars have been systematically investigated in the spectral range 50-2000 cm-1 at temperatures between 20 and 900 K. Samples with high degrees of Al-Si Order display IR absorption line profiles with spectral line widths that are systematically smaller than those for samples with low degrees of Al-Si order. Phonon frequencies depend in a nonlinear manner on the Na-K content, indicating nonideal mixing properties. The mode Grüneisen parameters are largest for low-frequency lattice modes and Si-O stretching modes; some negative Grüneisen parameters were found at phonon energies below 430 cm-1. The temperature evolution of the phonon spectra shows clear indications of the C2/m-C1̅ transition in Al-Si disordered Na-rich samples. An anomaly for highly Al-Si ordered microcline at 270 K is confirmed.

Lattice parameters, spontaneous strain and phase transitions in Pb3(PO4)2

The temperature evolution of the lattice parameters of lead phosphate, Pb 3 (PO 4 ) 2 , indicates three lattice instabilities with anomalies near 530, 453 and ca 430 K. Only the phase transition at 453 K is ferroelastic with a spontaneous strain e=(e 1 -e 1 0 0 e 5 0). Volume anomalies occur near 530 and ca 430 K that can be related to the multicomponent behaviour of the structural order parameter

Exsolution and Al-Si disorder in alkali feldspars; their analysis by infrared spectroscopy

Abstract An experimental method for the determination of Ab-Or exsolution and Al-Si ordering in alkali feldspar is described. Powder infrared spectroscopy was used to measure spectra in the spectral range between 50 and 1500 cm-1 using about 5 mg of sample material. The spectra were analyzed using reference spectra of uniform samples with various Ab-Or compositions and several degrees of Al-Si order. The application of this method is demonstrated for two examples of exsolved feldspar minerals, and it is shown that detailed characterization of alkali feldspars using IR spectroscopy leads to new insight into their structual details.

Kinetic rate laws derived from order parameter theory IV: Kinetics of Al,Si disordering in Na-feldspars

AbstractThe kinetic rate laws of Al-Si disordering under dry conditions (T = 1353K, 1253 K, 1223 K, 1183 K) and in the presence of water (p = 1 kbar, T = 1023 K, 1073 K, 1103 K) were studied both experimentally and theoretically. A gradual change of the degree of order was found under dry conditions. For intermediate degrees of order broad distributions of the order parameter Qod occur. The variations of Qod are correlated with structural modulations as observed in the transmission electron microscope. The time evolution of the mean value of Qod can be well described by the rate law:

Experimental test of rate equations: time evolution of Al,Si ordering in anorthite CaAl2Si2O8

\frac{{dQ_{od} }}{{dt}} = - \frac{\gamma }{{RT}}\exp \sum\limits_{i = 1}^n {X_i^2 } \left[ {\frac{{ - (G_a^0 + \varepsilon (\Delta Q_{od} )^2 )}}{{RT}}} \right]\frac{{dG}}{{dQ_{od} }}

Calibration of excess thermodynamic properties and elastic constant variations associated with the alpha beta phase transition in quartz

with the excess Gibbs energy G and Ga0= 433.8 kJ/mol, ɛ= -27.4 kJ/mol, γ = 1.687 · 1014 h −1.Under wet conditions, two processes were found which occur simultaneously. Firstly, some material renucleated with the equilibrium degree of order. Secondly, the bulk of the material transformed following the same rate law as under dry conditions but with the reduced activation energy Ga0= 332.0 kJ/mol and ɛ = -43.0 kJ/ mol, γ = 1.047 · 1013 h−1. The applicability of the kinetic theory is discussed and some ideas for the analysis of geological observations are evolved.

Landau free energy and order parameter behaviour of the α/β phase transition in cristobalite

AbstractThe lattice parameters of anorthites An98Ab2 and An100 have been measured from 22 to 1100 K. The spontaneous strain arising from the