Title page for ETD etd-12262007-110244

Type of Document Master's Thesis
Author Johnson, Eleda
Author's Email Address eledaj@vt.edu
URN etd-12262007-110244
Title The Elastic Behavior of Plagioclase Feldspar at High Pressure
Degree Master of Science
Department Geosciences
Advisory Committee
Advisor Name Title
Angel, Ross J. Committee Chair
Carpenter, Michael Committee Member
Ross, Nancy L. Committee Member
  • unit-cell parameters
  • X-ray diffraction
  • bulk modulus
  • equation of state
  • elasticity
  • plagioclase
  • pressure
Date of Defense 2007-12-07
Availability unrestricted
Feldspars are one of the archetypical families of framework silicates. They not only comprise around 60% volumetrically of the Earth’s crust, but are among some of the most structurally complicated minerals. Investigation into the structural behavior of various intermediate plagioclases at pressure has been undertaken with the intent of categorizing the elastic behavior with pressure across the solid solution series and establishing a conceptual model to characterize feldspar compression.

Complex behavior has been observed in the Equation of State for plagioclase feldspars in excess of 3 GPa, including an anomalous softening of ordered albite in excess of 8.4 GPa (Benusa et al 2005: Am Min 90:1115-1120). This softening was not observed in the EoS for the more intermediate plagioclase compositions containing between 20 and 40 mol% of end-member anorthite. The calculated elastic compliance tensor sums at room pressure show a general stiffening with increasing anorthite component, small elastic changes at the C-1 to I-1 transition, and a dominantly first-order response at the P-1 to I-1 transition near end-member anorthite.

The crystal structure of An37 plagioclase was determined by single-crystal X-ray diffraction. The compression mechanisms in An37 are similar to those in albite at lower pressures. The softening in albite at higher pressures is therefore attributed to the structural shearing in albite that is absent in An37 plagioclase up to 9.5 GPa.

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