Nicholas Moore

Compositional and Textural Analysis of Coexisting Aluminosilicate Polymorphs

This research focuses on the three aluminosilicate polymorphs (Al2SiO5): kyanite, andalusite, and sillimanite. Each polymorph is only stable in its own unique set of pressure and temperature conditions, so they provide important information about the pressure and temperature conditions of metamorphism. These minerals commonly occur in metamorphosed shale, but it is rare for all three to occur together. In rocks that contain all three polymorphs, one must be stable while the other two are metastable, and determining the order of crystallization helps decipher what metamorphic events occurred and when they occurred in relation to each other. The trace-elements, crystal structures, and reaction textures of coexisting aluminosilicate polymorphs are related to the pressure-temperature and deformation conditions of metamorphism. A clearer understanding of past conditions reveals the metamorphic events that drove the formation of the three polymorphs. We used sample suites of rocks containing kyanite, sillimanite, and andalusite from Norway and Iran. The samples were prepared into thin sections and then observed under a petrographic microscope to identify minerals and textures in the rock. Through these observations, we predicted that the crystallization sequence of both samples was andalusite first, followed by kyanite and ending with sillimanite. Next, the observed identities of the minerals were confirmed using electron backscattered diffraction. Then, cathodoluminescence (CL) imaging of the samples revealed some zoning of trace-elements within the minerals. We used the electron microprobe to determine mineral composition including trace-element abundances in the aluminosilicates, quartz, and rutile, which provided more information about metamorphic reactions.