Abstract
Potassium-calcium feldspar inclusions in a Bøggild 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 (C1̅ symmetry) and anorthite (P1̅ symmetry), whereas the core is unexsolved potassium feldspar (C1̅symmetry). Potassium feldspar and anorthite in the inclusions share the crystallographic orientation of the host Bøggild plagioclase. High-resolution lattice images indicate coherent interfaces between the inclusions and the host Bøggild 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 Bøggild exsolution lamellae → growth of anorthite on the rims of potassium feldspar inclusions. Potassium-calcium feldspar inclusions may be found in Bøggild and other plagioclase, and they have the potential to give constraints on thermal histories of K-rich plagioclase.
© 2015 by Walter de Gruyter Berlin/Boston