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Title: Order parameter coupling in framework silicate minerals
Author: Hayward, S. A.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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The Landau theory of phase transitions is used to model aspects of the behaviour of the minerals anorthoclase and nepheline. In both these minerals, there are multiple phase transitions, with separate time and temperature dependencies. The concepts of order parameter coupling and gradient energies are shown to give useful quantitative descriptions of these minerals. The displacive phase transition in anorthoclase is studied by X-Ray powder diffraction. The temperature dependence of the order parameter is shown to depend on the composition and the degree of noncovergent Al, Si ordering. These dependencies correspond with a simple Landau potential, where both the composition x and the degree of ordering Qt couple to the square of the displacive order parameter Q. At low temperatures, quantum mechanical saturation of order parameters occurs. The extent of saturation is characterised by a temperature θs, which is a function of the high frequency phonons in the structure. When a saturated phase transition is coupled with a classical effect, anomalous behaviour is expected in the phase diagrams. These anomalies are investigated for anorthoclases in the composition range Ab7Or30 to Ab55Or45. An analogous model is applicable to ferroelectric materials at low temperatures, such as KH2PO4 and SrTiO3. The role of local structural distortions in feldspar is analysed, where twin domains and solute atoms are used to distort the bulk structure. The wall thickness of a twin boundary is measured at various temperatures using single crystal X-ray diffraction. The thickness is 25 Å at room temperature, and increases on heating towards Tc. This is consistent with the Ginzburg-Landau model of fluctuations in the order parameter. The range of distortions due to solute atoms is found to be similar, as determined from anomalies in the temperature - composition phase diagram of anorthoclase.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available