TEM investigation of potassium-calcium feldspar inclusions in Boggild plagioclase

Abstract

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 (C symmetry) and anorthite (P symmetry), whereas the core is un¯¯ 11 exsolved potassium feldspar (C symmetry). Potassium feldspar and anorthite in the in¯ 1 clusions 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.