High-temperature molecular dynamics simulation of aragonite

Abstract

For molecular dynamics simulations using aragonite structure as the initial state, a new phase of spacegroup P6322 (hexagonal aragonite) appeared at temperatures above 510 K at a pressure of 1 atm. It was afirst-order phase transition which occurs metastably within the stable region of calcite and thedT/dP slope of the phase boundary between orthorhombic and hexagonal aragonite was about1.25 × 103 K GPa − 1. In the hexagonal aragonitestructure, CO3 groups wererotated by 30° around thec axis and move upand down along the c axis from their position in aragonite, and Ca ions were six-coordinated as they are in calcite. TheCaO6 octahedron of hexagonal aragonite was strongly distorted, whereas in thecalcite structure it is an almost ideal octahedron. The transition betweenhexagonal and orthorhombic aragonite involves only small movements ofCO3 groups. Therefore, it is possible that hexagonal aragonite plays an important part in themetastable formation of aragonite within the stability field of calcite and in thedevelopment of sector trilling in aragonite.