TEM investigation of potassium-calcium feldspar inclusions in Boggild plagioclase

University of Tokyo, Mineralogical Institute, Tokyo, Japan

Potassium-calcium feldspar inclusions in a Boggild plagioclase were investigated by analytical high-resolution transmission electron microscopy (HRTEM), and the formation process of these inclusions is considered based on the Al-Si ordering scheme in ternary feldspar system. Nanometer-scale chemical analyses and selected-area electron diffraction (SAED) patterns show that the rims of the inclusions consist of lamellar intergrowths of potassium feldspar (CI symmetry) and anorthite (PI symmetry), whereas the core is unexsolved potassium feldspar (CI symmetry). Potassium feldspar and anorthite in the inclusions share the crystallographic orientation of the host Boggild plagioclase. High-resolution lattice images indicate coherent interfaces between the inclusions and the host Boggild plagioclase and the phases in the inclusions. The results indicate a unique formation process of potassium-calcium feldspar inclusions in a ternary feldspar: crystallization of host labradorite-->precipitation of potassium feldspar-->decomposition of Boggild exsolution lamellae-->growth of anorthite on the rims of potassium feldspar inclusions. Potassium-calcium feldspar inclusions may be found in Boggild and other plagioclase, and they have the potential to give constraints on thermal histories of K-rich plagioclase.

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