Structural study of ellenbergerite. Part II: Effects of high pressure

Abstract

Compressibility and structural deformations induced by pressure in the high-pressure silicate ellenbergerite [Mg,(Ti,Zr),□] 2 Mg 6 (Al,Mg) 6 (Si,P) 8 O 28 (OH) 10 ; hexagonal, space group P6 3 , have been studied by single-crystal X-ray diffraction in a diamond anvil cell (DAC) in the pressure range 1 bar - 46 kbar. The bulk modulus of the mineral is 1330 kbar. The compressibility pattern is slightly anisotropic, with β a = 2.4 10 −4 and β c = 3.1 10 −4 kbar −1 . Compressibility is constant in the pressure range examined. Limited compositional differences of the samples do not produce significant variations in the compressibility coefficients. The geometrical data vs. pressure fit along the linear trends: a = a 0 - 0.0029 P c = c 0 - 0.0015 P with P in kbar and a and c in A. In agreement with its occurrence in coesite-bearing rocks, ellenbergerite is a very dense phase, with a structure formed by chains of face-sharing octahedra, which represent rigid structural moduli. Therefore the structural response to the high pressure applied in the DAC consists of small deformations in octahedral bond lengths and angles, with only minor interpolyhedral geometrical variations. Comparison between high-pressure and high-temperature data indicates that the unit cell and bond geometry of ellenbergerite at formation conditions, namely 700-800°C and 25-30 kbar, do not differ greatly from those at room conditions