Title page for ETD etd-05042012-170907


Type of Document Dissertation
Author Kolbus, Lindsay Marie
URN etd-05042012-170907
Title Structural variations of feldspars at high pressure and high temperature
Degree PhD
Department Geosciences
Advisory Committee
Advisor Name Title
Ross, Nancy L. Committee Chair
Angel, Ross J. Committee Member
Law, Richard D. Committee Member
Nestola, Fabrizio Committee Member
Slebodnick, Carla Committee Member
Keywords
  • feldspar
  • plagioclase
  • high-temperature
  • high-pressure
  • structure
Date of Defense 2012-04-24
Availability unrestricted
Abstract
Feldspar minerals are framework aluminosilicates that comprise approximately 60 percent of the Earth’s crust. The elastic and thermodynamic properties of this important mineral group are needed for the interpretation of seismic wave velocities, for understanding cation partioning patterns and for the determination of phase boundaries and reactions involving feldspars in the Earth’s crust. Until recently, no systematic approach has been applied to describe the structural behavior of feldspars as a function of pressure, temperature and composition. In this thesis, high-pressure and high-temperature X-ray diffraction data were collected for feldspars over a range of compositions which has led to the development a structural model that allows one to predict the structural evolution of feldspars at depth in the Earth’s crust. Specifically, the equations of state have been determined for two plagioclase feldspars (An20 and An78) with different states of Al/Si ordering using single-crystal X-ray diffraction. This study has shown that the introduction of Al,Si disorder into plagioclase structures at constant composition softens the structure by 4(1)% for An0, 2.5(9)% for An20 and is essentially zero for An78 compositions. The effect of pressure on the structure of an ordered An20 was also determined up to 9.15 GPa using single-crystal X-ray diffraction and it was found that the dominant compression mechanism involves tilting of the AlO4 and SiO4 tetrahedra. Similarly, high-temperature single-crystal X-ray diffraction data collected from an ordered An26 plagioclase and powder X-ray diffraction collected on a suite of Na-rich plagioclases that were refined using the Rietveld method indicate that the major structural response to increased temperature involves tilting of the tetrahedra. Building on ideas originally proposed by Dr. Helen Megaw, the changes in the conformation of the tetrahedral framework of feldspars can be described in terms of four distinct tilt systems of rigid tetrahedra. This model demonstrates that the fundamental reason for the observed anisotropy and volume change of feldspars lies in the topology of the tetrahedral framework with the greatest contribution attributed to tilt systems 2 and 3.
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