Exsolution and coarsening mechanisms and kinetics in an ordered cryptoperthite series

Abstract

Cryptoperthites from the Klokken layered syenite intrusion were examined by TEM to determine the role of exsolution, ordering and twinning in the development of the coherent microtextures during slow cooling, the stratigraphic position of the samples in the layered series giving an independent variable in determining their evolution. Both periodicity (primary and secondary) and morphology change with distance from the top of the series. Most samples contain low microcline in the diagonal association.

Partial ordering occurred before exsolution, which was followed by Albite-twin formation in the albite lamellae. The twin periodicity depends on the average lamellar thickness (or on the primary lamellar periodicity, λ ₁) and no longer changes during subsequent morphological evolution. In the Or-rich lamellae long-period Albite twins develop before waves form in the lamellar interface. The interfaces rotate with increasing order to give parallel-sided zig-zag lamellae of low microcline with Albite twinned lamellae of low albite, which may pinch and swell. Where the albite lamellae are discontinuous, adjacent microcline lamellae coalesce giving oblique lamellae and Pericline or ‘M-type’ twins. Thickening of some oblique lamellae gives a distinct secondary periodicity, λ ₂, which outlines lozenge-shaped areas with relics of the primary periodicity and, if coarse enough, is responsible for optically-visible braid microperthite. Coherency, demonstrated by high resolution images, is maintained through all stages of the coarsening.

A time-temperature-transformation diagram for continuous cooling is presented and can be used to interpret the kinetics and morphological evolution of cryptoperthites from rocks with very different cooling rates (dykes and lavas to very large plutons), which have, however, similar primary lamellar periodicities. The finest periodicities are only slightly larger than the supposed initial periodicities (λ _o) for spinodal decomposition and little coarsening can have occurred. Coarsening at cooling rates slow enough to produce significant ordering may be much slower than coarsening in disordered feldspars. Primary coarsening may be stopped by the development of Albite twins in the Abrich phase, which will require reversal of the order-antiorder sense of parts of the framework. Coarsening may also be slowed if the phases at intermediate temperatures order at different rates or have different equilibrium degrees of Al-Si order. Secondary coarsening can develop at much lower temperatures (<400° C) on the formation of low microcline, when both phases have the same framework order.