The high albite–monalbite and analbite–monalbite transitions

The temperatures T_diff and T_displ of the diffusive transformation high albite–monalbite and the displacive transformation analbite–monalbite were investigated by X-ray powder methods. Alkali feldspars of composition Or_0,5,10,20,30 were prepared by hydrothermal synthesis at temperatures T_equil between 1060 and 700°C. Run durations were sufficiently long (3–72 days) to allow the samples to reach Al, Si equilibrium distributions. T_diff can be represented by the equation T_displ[^oC] = 978 — 19.2 · Or, whereas T_dispi also depends on T_equili.e. the degree of Al,Si order, according to T_displ[°C] = 715 – 18.9 · Or – 0.221 · Or² + 0.269 · T_equil. As a consequence Or_displ, the composition of the triclinic/monoclinic inversion at room temperature, also changes with T_equil : Or_displ[mol%] = 27.5 + 0.00842 ·T_equil

Distinction between topochemically triclinic and monoclinic symmetry is possible on the basis of the temperature variation of lattice angles α, γ and simple line separations; e.g. cos²α of high albite smoothly approaches but never reaches zero with increasing temperature, whereas cos²α of analbite is consistent with a straight-line extrapolation to zero. At room temperature an interval of uncertain distinction extends from γ = 90.18 to 90.25° and from Δ131 = 1.95 to 2.00 [±2θ] in high albite and analbite.

Structural explanations are given (1) for the variations of α and γ with Al,Si order in equilibrated albites, (2) for the variations of α and γ due to short-term heating of albite and analbite, (3) for the dependence of T_displ and Or_displ on T_equil, (4) for the result T_displ ≥ T_diffin Na-feldspar. T_displ is thought to react sensitively on structural disturbances which may be caused by thermal and mechanical history, chemical impurities, and other factors. If T_displ is found to be lower than T_diff, this is ascribed to such defects.