Proto-polymorphs of jimthompsonite and chesterite in contact-metamorphosed serpentinites from Japan

Proto-polsemorphs 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.