JIM THOMPSON AND THE BIOPYRIBOLE POLYSOMATIC SERIES

At an informal seminar on the crystal chemistry and natural occurrence of amphiboles at Univ. of Massachusetts in March, 1968, Jim Thompson presented, for the first time outside his classrooms, what would prove to be a powerful concept relating amphibole crystal structures to those of pyroxenes and micas. He pointed out that the amphibole structure (MP) could be constructed of alternating (010) slabs of pyroxene structure (P) and sheet silicate (talc or mica) structure (M). Throughout the early 1970’s Jim expanded and generalized the concept of so-called “modular” structures, coining the term “polysome” to apply to a crystal made up of chemically and structurally distinct units. Pyroxenes, amphiboles and micas thus represent a “polysomatic series.” If amphiboles are an alternation of M and P slabs in a 1:1 ratio, why, Jim asked, could not other ratios and sequences of M and P exist?

At Jim’s suggestion, my graduate student at the time, David Veblen, visited the Chester, VT, talc quarry hoping to find specimens of pure anthophyllite, for reasons related to questions about possible symmetries of orthorhombic amphiboles. David’s single-crystal X-ray examination of a crystal of what we expected to be anthophyllite showed – surprisingly – a structure with slab sequence MPMMP, the first known mixed-chain silicate, named chesterite after the locality. Further exploration of other crystals from his bulk sample yielded yet another new biopyribole, a triple chain phase, MMP, named jimthompsonite for obvious reasons. These discoveries unequivocally answered Jim’s question: Yes, the biopyribole polysomatic series is more richly populated than previously recognized.

These complex biopyriboles are most frequently intergrown on a fine scale, suggesting they are intermediate products in reactions from amphiboles to micas. High-resolution transmission electron microscopy has revealed extensively disordered examples with extra or missing M slabs as well as multiple repeats of more complex slab sequences with essentially zero probability of occurring randomly.

The concept of polysomatism has thus significantly enriched our crystal chemical understanding of biopyriboles. This is but one example of Jim Thompson’s extraordinary imagination and insight, the fruits of which will benefit mineralogy for the foreseeable future.