The average structure of An 62–66 labradorite

Abstract

The paper reports results of X-ray and neutron structure refnements of four labradorites An 62-An 66 with di-fferent geological histories and di-fferent superstructures. A rapidly quenched crystal collected during the eruption of Surtsey (Iceland) is essentially disordered. Labradorite from massive lava flows of Lake County (Oregon) shows D reflections, and two metamorphic labradorites from the Central Alps display an intermediate plagioclase structure with e reflections. The average structure (c : 7 A) of all four samples converging at R factors of 3-6Vo are virtually identical with well-defined Si/Al and smeared-out oxygen positions. We interpret this as evidence that the di-fferent superstructures consist of periodic stackings of similar basic units probably lssepfling those in 1l anorthite but with diflerent stacking orders. The main intensity contribution to superstructure reflections comes from displacements of oxygens in the superstructures, probably caused by Al-Si ordering. Phase relations are illustrated in a hypothetical TTT diagram in which we summarize results from structure refinements, microstructural observations and changes during experimental heating. At high temperature Qittle undercoqling below the critical temperature) ordering takes place by a nucleation and growth mechanism which produced 1l domains separated by curved APB's (antiphase boundaries) (Lake County). At larger undercooling continuous ordering prevails, giving rise to periodic APB's and satellites in the diffraction pattern.