The anorthoclase structures: the effects of temperature and composition

Cell-parameter and structure refinements using single-crystal X-ray diffraction have been carried out on three natural low-Ca anorthoclases (K–analbites), (Or_32.5Ab_66.7An_0.8, Or_22.3Ab_70.8An_6.9, Or_13.8Ab_83.7An_2.5), at room temperature and at temperatures above the triclinic/monoclinic symmetry inversion (400°, 510°, and 750° C, respectively). Room temperature cell parameters are consistent with monoclinic Si,Al topochemistry and a high degree of disorder (t₁ = 0.50 to 0.52 from the b–c plot). A room-temperature inversion composition of Or₃₆ is extrapolated from cos²α* and T–O–T angle plots. Mean M–O bond lengths and T–O–T angles increase with potassium content, whereas their variances decrease, indicating a trend to more regular coordination, as in sanidine. A similar relationship develops with thermal expansion and the structural changes associated with inversion to monoclinic symmetry, but the individual (M–O) values suggest a nonisotropic, ellipsoidal expansion of Na-rich alkali cavities. Examination of dimensional change in the structures due to change in temperature and alkali composition indicates that most of the effect is related to change in M–O bonding, with the aluminosilicate framework responding passively. Anisotropic thermal models for single M sites produce non-positive definite ellipsoids. Modeling with isotropic split-sites yields four parts for the more sodic structures, essentially identical to the models for high albite or analbite. For the more potassic crystals, a separate isotropic K site and an anisotropic Na(Ca) site is indicated.