Biological Minerals Formed from Strontium and Barium Sulphates. II. Crystallography and Control of Mineral Morphology in Desmids

J. R. Wilcock, C. C. Perry, R. J. P. Williams, A. J. Brook

Abstract

The crystallography and morphology of barium sulphate crystals produced by a range of desmid species have been examined by electron microscopy and electron diffraction. The morphology of the BaSO$_4$ crystals varies between desmid species and water chemistry Tabular rhombic crystals are commonly produced by Closterium lunula and hexagonal or elliptical crystals by Micrasterias thomasiana. High-resolution transmission electron microscopy (HRTEM) has confirmed the single-crystal nature of the biological precipitates. The crystals all lie with their tabular faces perpendicular to the [001] zone axis and are comparable with the typical crystal habit of geological and synthetic barium sulphates. suggesting a limited degree of biological control over precipitation. M$^{2+}$ (Ba$^{2+}$ or Sr$^{2+}$) SO$^{2-}_4$ ion ratios in vivo and in vitro have been shown to affect the morphology of the crystals produced. For Closterium lunula an M$^{2+}$. SO$^{2-}_4$ ion ratio of less than 1 1 produces rhombic crystals and an ion ratio of more than 10 1 produces hexagonal crystals. For Micrasterias thomasiana. an ion ratio of 0.01 1 produces hexagonal crystals. an ion ratio of 1 1 produces a mixture of hexagonal and elliptical crystals, and an ion ratio of more than 10 1 produces elliptical crystals. Crystal morphologies produced by desmids have been compared with those of synthetic crystals produced at similar ion ratios. The apparently limited extent of biological control and the importance of the ionic environment on the development of crystal morphology are discussed. It is proposed that desmid crystal morphology may provide a diagnostic for the local environment of crystal formation in the cell.