Pathological feldspar crystals



Zoned plagioclase crystals have long been used as a petrologic tool for marking magma fractionation trends. In the early part of this century, N.L. Bowen solved many of the riddles of feldspar zoning and of even reverse zoning. Now, C.J. Allegre et al. [Oscillatory zoning: A pathological case of crystal growth, Nature, Nov. 19, 1981] proposes a new theory on a secondary feature of plagioclase crystal zoning, namely, it is an explanation of the oscillatory character of the zoning of many naturally formed plagioclases.

The results reported by Allegre and colleagues reflect the increasing number of studies being done on igneous melts in the laboratory. The model proposed takes into account the coupling between ‘interface kinetics’ and the diffusion of chemical species in the melt. In Allegre's examples, plagioclase zones oscillate back and forth rather uniformly in varying albite content, as one tracks across a zoning profile from the crystal center to its edges. The theoretical analogy is described as ‘the perturbation equations result in almost undamped, dissymmetric oscillations in the concentration profile.’ The growth rate equations developed in the study predict a given episode of oscillatory zoning in all cases where the supersaturation of a given crystallizing-diffusing component is not large.