Abstract

Proto-polymorphs of jimthompsonite and chesterite occur in metamorphosed serpentinites from two Japanese ultramafic complexes. The lattice constants of the proto triple-chain silicate, measured by X-ray diffraction, are a = 0.93605(208), b = 2.72560(588), and c = 0.53160(89) nm, whereas those of the mixed double- and triple-chain silicate are a = 0.94202(78), b = 4.54402(392), c = 0.53440(45) nm, and β = 90.026(18)°. The lattice constants and systematic extinctions revealed by selected-area diffraction patterns are consistent with proto-triple-chain silicate (Pbcn) and mixed double- and triple-chain silicate (A2/m, Am), but not with the ortho- and clino-polymorphs. High-resolution transmission electron microscopy [016] images of the triple-chain silicate and [0 1 15] images of the mixed-chain silicate indicate they have a (•X) configuration. Proto forms of wide-chain pyriboles might be geologically widespread.

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