BIOPYRIBOLES: A RETROSPECTIVE (Invited Presentation)

Although perhaps best known as a petrologist, Jim Thompson thought himself equally a mineralogist. In addition to fundamental work on feldspars, his theoretical contributions to the crystallography and chemistry of chain and sheet silicates are profound: his beloved biopyriboles (a term borrowed from Johannsen indicating the group encompassing biotite, including all micas, talc, and pyrophyllite; pyroxenes; and amphiboles). His detailed explorations of stacking variations and chain rotations in pyroxenes and amphiboles spawned a cottage industry in theoretical, model pyriboles.

Thompson’s most novel and insightful studies of biopyribole structure and chemistry arose from the observation that alternating (010) slabs of talc or mica and pyroxene (MP) produce the amphibole structure. He named structures constructed from interleaved slabs of two or more structures polysomes; a series of such structures, such as the biopyriboles, a polysomatic series; and this mixed-structure phenomenon polysomatism. Upon learning of the discovery of the first naturally occurring triple-chain silicates, Thompson pulled out several completed manuscripts, including one that detailed the biopyriboles as a polysomatic series and predicted that other polysomes might be discovered, such as (MMP). This is a triple-chain structure, and the orthorhombic and monoclinic stacking variants were named jimthompsonite and clinojimthompsonite to honor his prediction.

This approach also can be used to explain mixed-chain silicates containing both double and triple chains, as in chesterite (MPMMP), and a synthetic high-pressure phase with alternating single and double chains (MPP). Streaking on X-ray diffraction patterns of jimthompsonite and chesterite suggested that there might be structural disorder parallel to [010]. Subsequent studies using high-resolution transmission electron microscopy confirmed that biopyriboles can contain an exceedingly rich collection of crystallographic irregularities. Many of these structural complexities involve polysomatic defects that can be described as disordered sequences of silicate chains (or aperiodic sequences of M and P). Such disorder can provide insights into the processes whereby minerals are changed, nominally in the solid state, during metamorphism and weathering.