Doctor of Philosophy
Shieh, Sean R.
Osinski, Gordon R.
Plagioclase feldspar is one of the abundant minerals on the surface of the Earth, the Moon and Mars, and is also commonly found in meteorites. This dissertation seeks to better understand the phase changes in plagioclase when subjected to meteorite impact and fundamental information about impact cratering process.
Shock features in plagioclase from Mistastin Lake impact structure, Canada, and Apollo samples returned from the Moon were investigated. A series of progressive optical shock features were observed, and their Raman spectra was collected systematically. Identified shock featurs include deformed twins, planar features, and partially to completely isotropic or diaplectic glass. However, no planar deformation features (PDFs) were observed in plagioclase despite finding PDFs in quartz in the same thin section in Mistastin Lake central uplift samples. Certain features observed in plagioclase (e.g., twinning, needle like inclusions, zeolite alteration and crystalline planar features) may be mistaken as PDFs but can be easily identified by their Raman features. Raman spectra of shock features show that Raman features change progressively as shock level increases. Raman peak broadening, reduction of peak intensities, and peak loss were observed indicating structure deformation, loss of crystallinity and amorphization. Crystalline plagioclase and amorphous diaplectic glass can be distinguished based on their Raman features.
To better understand the deformation of plagioclase under high temperature and high-pressure conditions, static experiments were also conducted using laser heating diamond anvil cell and synchrotron X-ray diffraction. Results reveal pressure/temperature-induced amorphization and crystallization kinetics of high-pressure phase changes. The phase diagram of intermediate plagioclase was constructed up to 64 GPa and 2000 K and a new high-pressure phase was observed. This phase diagram reveals a wide pressure-temperature range for plagioclase to change from crystalline to amorphous state (i.e. diaplectic glass) before melting. Furthermore, the breakdown and phase changes make plagioclase an effective barometer candidate for estimating the pressure-temperature history of shock events and interpreting the mineralogy of meteorites, ultimately of great help to reconstruct the collisional history of asteroids in the early Solar System.
Summary for Lay Audience
Feldspar, a grey or pink coloured mineral, is frequently seen in kitchen countertops, and commonly found in gift shops with a blue tint, where it is known as moonstone or labradorite. However, feldspar is not just a pretty stone for decoration but is also makes up over 50% of Earth’s crust, where we are standing on right now. But how did we get here? Scientists believe that some meteorites brought the seeds of life to Earth, same as how they killed the dinosaurs, and feldspar might be a witness to this impact process! This study is thus motivated to solve the case by studying feldspar samples collected from impact structure on Earth, and samples returned from the Moon where feldspar is also commonly impacted.
Feldspar samples were investigated using a variety of modern techniques. The primary technique is called Raman spectroscopy and it can identify chemical properties and inner features within the mineral. Also, feldspar samples were compressed between two diamonds to experience a controlled pressure and temperature conditions. By using super bright X-rays, changes of inner features and the development of new features were monitored.
The results of this study revealed a variety of changes within feldspar due to the impact process and they can be identified and classified by their Raman features. The data from lab experiments helped to explain the development of these features and to scale the pressure and temperature conditions they experienced. Ultimately, this study will help people understand the formation of Earth, the Moon and our solar system.
Xie, Tianqi, "Raman Spectroscopy and Synchrotron X-ray Diffraction Study of Lunar and Terrestrial Plagioclase Feldspar at High-Pressure and High-Temperature Conditions" (2020). Electronic Thesis and Dissertation Repository. 7519.
Available for download on Friday, December 31, 2021