Albitisation of plagioclase in nature and experiment

Metasomatic albitisation of feldspar is a relatively common phenomenon in a range of geological environments, from diagenetic to hydrothermal, and involves the replacement of a pre-existing feldspar by virtually pure albite. Albitisation fronts can be recognised on a large scale in the field, as in the albitite terrains of the Bamble sector SE Norway and the Curnamona Province, South Australia, but also can be identified within single crystals on a nano-scale. In the field, albitisation fronts may extend nornal to fractures and are sometimes recognised by a reddening of the feldspars due to nano-inclusions of hematite which form in the albite. To determine the mechanism of albitisation within single crystals of plagioclase (An21-23), intragranular replacement interfaces have been studied by electron probe microanalysis, scanning (SEM) and transmission electron microscopy (TEM). The albite replacement product (An-2-5-) has micron-sized pores observable by SEM as well as nanopores imaged by TEM. The albite contains fine-grained mica (sericite), often associated with pores, as well as precipitates of hematite. The chemical interface between the plagioclase and albite is sharp on the scale of tens of nanometres, and the crystallographic orientations of plagioclase and albite are coincident within less than a degree.
Experimental replacement of plagioclase (oligoclase and labradorite) by albite can be readily achieved within days by hydrothermal reaction in a saline, silica-bearing fluid. Experiments on oligoclase at 600OC and 2 kbars result in reaction rims of albite with very similar characteristics to the naturally albitised feldspar. The observed features are all characteristic of an interface-coupled dissolution-reprecipitation replacement mechanism, which in combination with fracturing allow fluid infiltration and mineral replacement.