The high—low albite relations revealed by reversal of degree of order at high pressures

The equilibrium Al/Si order–disorder relations in albite have been clarified by reversal of the degree of order at high pressures (~18 kbar) in the absence of water to avoid melting, and for the most part in the presence of essentially anhydrous carbonates. At high pressures the Al/Si diffusive interchange proceeds orders of magnitude faster than observed at 1–2 kbar P(H₂O). At 18 kbar albite begins to disorder above 660°C, and the Δ2θ(131) indicator of order goes from 1.10 at 660°C to 1.85 (high albite) at 790°C. The rate of change is 9.1 × 10⁻³ Δ/K in this region of stable intermediate albites. At temperatures from 790°C to 950°C disorder in high albite continues essentially linearly with temperature from Δ = 1.85 to Δ ~ 2.0, but at an order of magnitude lower rate of 9.2 × 10⁻⁴ Δ/K. The pressure effect is calculated at 350 bars/K, and experimental data confirm this value; thus equilibrium order—disorder temperatures should be 45–50°C lower at 2 kbar than at 18 kbar.

Structural state modification is dominantly by the solid-state diffusion of Al and Si. Although both hydrothermal and carbonate-containing runs show some evidence of recrystallization, neither added water nor carbonate appear to be essential. For example, relatively rapid ordering or disordering takes place in the absence of fluxes at temperatures of 750–1000°C in albite dried at 1000°C for 24 hours prior to treatment and in albites treated without carbonate at pressures just below the albite → jadeite + quartz transition. It is tentatively suggested that the observed great enhancement of diffusion at high pressures is the result of activation produced by transient higher coordination induced around Al atoms, with concomitant disruption of the linked Si–O units. This process may be aided by small amounts of water or hydrogen.

The low ⇄ high albite conversion takes place without change in symmetry (C1) over several hundred degrees, and if it can be called a phase transformation, it appears not to be first order in nature, at least up to the transition to the monoclinic modification, monalbite, at approximately 950°C.