The high-pressure crystal chemistry of low albite and the origin of the pressure dependency of Al-Si ordering

Structural and volume compressibility data for low albite were obtained by single-crystal X-ray diffraction methods at pressures up to ~4 GPa. The bulk modulus was determined to be 54(1) GPa, with a pressure derivative of 6(1). Intensity data were collected, and structures were refined at 0.00, 0.44, 1.22, 2.68, and 3.78 GPa. With increasing pressure, (1) the volumes of the TO₄ tetrahedra do not vary, (2) the volume of the NaO₇ polyhedron varies linearly with the volume of the unit cell and (3) Si-O-Si angles increase or remain constant, but only Al-O-Si angles decrease, which is consistent with the smaller force constant of the Al-O-Si vs. Si-O-Si angle. It is concluded that compression is accomplished through the bending of Al-O-Si angles, which squeezes together the chains of four-membered rings that run parallel to [001] and that are separated by zigzag channels containing Na atoms. -from Authors