Microstructures and reaction mechanisms in biopyriboles

High-resolution transmission electron microscopy of ferromagnesian chain and sheet silicates from a metamorphosed ultramafic body at Chester, Vermont has revealed a wide variety of structural defects. Images of these microstructures have been interpreted by analogy with images of ordered chain silicates and with the aid of dynamical diffraction and imaging calculations. Most of the defects are concentrated in regions of chain-width disorder; they apparently result from retrograde reaction of anthophyllite to talc and from deformation of the ultramafic body.

The most common defects are the terminations of (010) slabs having a given chain width. These slabs, referred to as “zippers,” in most cases terminate coherently, with no displacive planar defects associated with the termination. Two theoretically-derived rules must be obeyed for coherent termination. Rule 1: the terminating zipper must have the same number of subchains as the material it replaces. Rule 2: the numbers of silicate chains in the zipper and in the material it replaces must both be even, or they must both be odd. Where these rules are disobeyed, zipper terminations are usually associated with planar faults having displacements projected on (001) of either ¼[010] or ¼[100], referred to an anthophyllite cell. In rare cases, violation of the replacement rules results in severe structural distortion, rather than the creation of displacive planar faults. These replacement rules hold for all pyriboles and may be controlling factors not only in reactions involving amphibole, but in pyroxene hydration reactions as well. Other observed microstructures in the Chester chain silicates include cooperative terminations of many zippers; terminations of individual silicate chains; column defects parallel to the pyribole c axis; narrow lamellae of monoclinic pyribole; and extended displacive faults and rotational faults that apparently result from physical deformation.

Two types of talc have been distinguished by their orientation relationships with the pyriboles. In “coherent talc,” the relationship is b_tc||b_py, a_tc||c_py, c_tc||a_py. Structural considerations and high-resolution images suggest that this type of talc possesses 2O stacking. In “(210)-talc” there are no rigid orientation relationships with the intergrown pyribole, but the sheets of the talc tend to be parallel or subparallel to (210) of the pyribole. This type of talc displays extreme stacking disorder.

Textural evidence suggests that the wide-chain silicates and fibrous talc at Chester formed by retrograde hydration reaction of anthophyllite. This conclusion is supported by the microstructures observed with electron microscopy. These microstructures suggest that there were several mechanisms for the reaction: (a) anthophyllite and wide-chain pyribole were replaced along grain boundaries and fractures by talc, and (b) anthophyllite was replaced by a far more complex mechanism involving the reaction sequence anthophyllite → disordered chain silicate → chesterite → more disordered chain silicate → jimthompsonite → talc or disordered chain silicate with very wide chains. The disordering steps of this sequence apparently occurred by the nucleation and growth of wide-chain zippers. When a region of crystal achieved a chemical composition close to that of one of the ordered phases, reordering occurred by the passage of en echelon termination faults through the structure in the a direction. Much of the chemical transport necessary for these reactions may have taken place by “ultrafast” diffusion along large structural tunnels associated with zipper terminations.