Equation of state and structure of prehnite to 9.8 GPa

The equation of state and high-pressure behaviour of a natural sample of prehnite from Kuruman, South Africa, Ca2.07(Al0.98,Mn0.02)(AlSi3.09O10)(OH)2, have been investigated using single crystal X-ray diffraction up to 9.75(3) GPa. A second-order Birch–Murnaghan equation-of-state fit to the isothermal P - V data below 8.7 GPa yields values of K 0T = 109.29(18) GPa and K ′ = 4. A third-order Birch-Murnaghan equation fit to the P - V data results in K ′ values equal to four, within an estimated standard deviation. The compressibility of a, b , and c unit-cell parameters is non-linear and strongly anisotropic, with room-pressure moduli of K a 0 = 83.5(7) GPa, K b 0 = 113(1) GPa, and K c 0 = 146(3) GPa and K ′ a 0 =4.5(3), K ′ b 0 = 1.3(3), and K ′ c 0 = 8.8(8). Intensity data were collected over the entire pressure range. Bond lengths and angles, refined in the average space group Pncm , suggest that the structure compresses uniformly. Above 8.7 GPa there is an additional softening of the volume and the b -axis, but the average structure is maintained across the transition.

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