Spherulitic textures in glassy and crystalline rocks

Spherulites are clusters of confocally radiating crystals that, contrary to the implication of their name, are not commonly spherical but usually resemble sheaves of wheat tied at the center. The variation of morphology from the wheat sheaf to the spherical spherulite and the transition of growth conditions from the spherulite to the single dendrite and then to the more equant polyhedral crystal is well explained by a theory of spherulite growth developed and experimentally verified for high polymers. The stability of dendrites or spherulites during growth depends on the accumulation of a lower temperature component than the growing phase (impurity) at the interface of that growing crystal. The presence and dimensions of this layer are functions of the diffusion rate of the impurity and the growth rate of the crystal; spherulite- and dendrite-forming melts must have the proper ratio of these two variables. To stabilize multicrystalline spherulites, the impurity layer must be sufficiently small. As the width of this layer increases or as the diffusion rate more closely approaches the growth rate, the transition to dendritic or fibrous single-crystal growth, and ultimately to the growth of more equant polyhedral crystals, is observed. The presence of spherulites or dendrites in crystalline or glassy material therefore indicates that a given silicate melt or glass has existed in the range of physical conditions that, for a particular composition, is suitable for the growth of these fibrous crystal forms. A single-rock specimen, lunar sample 12009, contains all growth forms, equant phenocrysts, phenocrysts with dendritic projections, feathery dendritic single crystals, and spherulites. The presence of these crystal forms does not uniquely define the mode of formation of the original material, for example, as formed by shock-melting processes.