Synneusis Reconsidered

Abstract

The principal evidence advanced in favor of synneusis (swimming together and attachment of crystals in igneous melts) is the observation in thin section of concentric zoning, with the centers of zoning in the individuals offset from the mutual boundary. However, randomly oriented sections through normal growth twins (twinned nucleus) and epitaxial overgrowths can show such offset in a large percentage of cases. A further possible cause of offset is the infilling of skeletal, dendritic or spherulitic growth forms. Irregularity and asymmetry, which have been cited as evidence of synneusis origin, are normal in epitaxial overgrowths, and may be displayed even in perfectly symmetrical growth twins, because of cuteffects and the fact that mismatch of faces at the composition plane of rotation twins often causes overlapping growth.

Experiments have shown that crystals in fluids can be aligned, apparently by hydrodynamic forces, so that gross shape elements are parallel and in contact; and that once in position, crystals can become attached. However, the twin laws observed in feldspar are not those which would be predicted on the basis of gross-shape alignment, but are highly selective in crystallographic relations. There is abundant evidence that supposed synneusis clusters occur early in crystallization, whereas crystal-crystal contacts should increase in frequency with the concentration of crystals. Forces which could cause crystal faces to be directly attracted to one another over distances of greater than a few atomic diameters are at present unknown.

Essentially all heretofore observed features of crystal clusters, especially those of early clusters, can be explained more simply as the result of growth processes than as the result of synneusis.