Abstract

Retrograde hydration of anthophyllite found in ultramafic lenses of the Lepontine Alps, southern Switzerland, resulted in the formation of talc and additional ordered and disordered biopyribole polysomes. Among the ordered sequences, 13 new short-range biopyribole polysomes were discovered using high-resolution transmission electron microscopy (HRTEM). The most significant new polysomes are ordered sequences of quadruple-, triple-, and double-chains: (42), (432), (4323), and the first pyribole polysome with a composition between pyroxene and amphibole, (2111). Up to 40 unfaulted repetitions of these sequences are present in several grains. More complex polysomes like (423232) occur as intermediate steps in the formation of shorter polysomes, e.g., the polysome (332). The polysome (42) is the first short-range polysome, for which each step of the formation mechanism is documented. The precursors are disordered sequences countering quadruple- and double-chains, which are formed through coherent zipper transformations from anthophyllite. The disordered sequences are subsequently transformed to the ordered polysome (42). The multiple-step reaction path strongly suggests that energy minimization is partly responsible for the appearance of some of the short-range polysomes.

The major element chemistry of the short-range polysomes, determined by analytical electron microscopy (AEM), is identical to that of the macroscopically occurring biopy-riboles, excluding, therefore, a stabilization of the short-range polysomes by additional elements.

The ordering of (010) stacking faults in enstatite connecting partial dislocations, each with a ¼[100] ± ⅓[001] displacement component, led to the formation of the (2111) polysome. Along the fault plane, pairs of pyroxene I beams are juxtaposed and transform subsequently to amphibole chains. The composition of the (2111) polysome is colinear with the coexisting enstatite and anthophyllite.

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