First-Principles Study of the Structural Evolution of Microcline Under High Pressure

Microcline is an important rock-forming mineral in the Earth’s crust, and characterizing its structural behavior under compression is essential for understanding the high-pressure response of feldspar minerals under geological conditions. In this study, the crystal structural evolution of microcline up to 12 GPa was investigated using first-principles calculations based on density functional theory. The results reveal an isosymmetric phase transition at approximately 6–7 GPa, accompanied by a ~7% volume collapse. Across this transition, the b-axis and unit-cell angles (α, β, γ) change abruptly, and the aluminum coordination transforms from fourfold to a distorted fivefold geometry intermediate between a trigonal bipyramid and a square pyramid. Analysis of bond lengths and angles indicates that compression in the low-pressure phase is primarily driven by shear deformation of tetrahedral Ring 1. Near the transition pressure, however, marked shear deformations of Ring 2 and Ring 3 induce a strong contraction of the b-axis and abrupt changes in the unit-cell angles. Comparison with the compression behavior of low albite reveals both similarities and distinct structural responses, highlighting the role of framework topology and extra-framework cations in controlling pressure-induced structural evolution in feldspar minerals. These results provide new insights into the high-pressure behavior of microcline and contribute to a better understanding of the structural stability of feldspar minerals in the Earth’s interior.