The high-pressure crystal chemistry of low albite and the origin of the pressure dependency of Al-Si ordering
1994; Mineralogical Society of America; Volume: 79; Linguagem: Inglês
ISSN
1945-3027
AutoresRobert T. Downs, Robert M. Hazen, L. W. Finger,
Tópico(s)Clay minerals and soil interactions
ResumoStructural 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). Unit cell compression is anisotropic, as indicated by unit strain tensors. In the softest direction, approximately perpendicular to (100), the structure is three times more compressible than in the stiffest direction. 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 T04 tetrahedra do not vary, (2) the volume of the NaO? polyhedron varies linearly with the volume of the unit cell, and (3) Si-O-Si angles increase or remain constant, but only AI-O-Si angles decrease, which is consistent with the smaller force constant of the AI-O-Si vs. Si-O-Si angle. We conclude that compression is accomplished through the bending of AI-O-Si angles, which squeezes together the chains offour-membered rings that run parallel to [001] and that are separated by zigzag channels containing Na atoms. The feldspar three-dimensional tetrahedral framework can be considered to be made up of these chains, which are linked together by Octype atoms. The average value of the T-Oc T angle correlates with bulk moduli of alkali feldspars. Al-Si disorder tends to stiffen the T -Oc- T angle in high albite, which in turn decreases the compressibility and thus can serve as a mechanism for pressure-dependent ordering of high to low albite.
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