American Mineralogist, Volume 65, pages 599-623, 1980 Microstructures and reaction mechanismsin biopyriboles Devro R. VnnreN AND PETERR. BusEcK Departmentsof Geologysnd Chemistry,Arizona State University Tempe,Arizona 85281 Abstract High-resolution transmission electron microscopy of ferromagnesian chain and sheet sili- cates from a metamorphosed ultramafic body at Chester, Vermont has revealed a wide vari- ety of structural defects.Images of thesemicrostructures have been interpreted by analogy with images of ordered chain silicates and with the aid of dynamical diflraction and imaging calculations.Most of the defectsare concentratedin regionsof chain-width disorder;they ap- parently result from retrogradereaction ofanthophyllite to talc and from deformation of the ultramafic body. The most common defects are the terminations of (010) slabs having a given chain width. Theseslabs, referred to as "zippers," in most casesterminate coherently,with no displacive planar defects associated with the termination. Two theoretically-derived rules must be obeyedfor coherenttermination. Rule l: the terminating zipper must have the samenumber of subchains as the material it replaces.Rule 2: the numbers of silicate chains in the zipper and in the material it replacesmust both be even, or they must both be odd. Where these rules are disobeyed, zipper terminations are usually associatedwith planar faults having dis- placementsprojected on (001) of either % [010] or t/+[fi)], referred to an anthophyllite cell. In rare cases,violation of the replacement rules results in severestructural distortion, rather than the creation of displacive planar faults. These replacement rules hold for all pyriboles and may [s s6ahqlling factors not only in reactionsinvolving amFhib,ole,but in pyroxene hydration reactions as well. Other observed microstructures in the Chester chain silicates in- clude cooperative terminations of many zippers; terminations of individual silicate chains; column defects parallel to the pyribole c axis; nanow lamellae of monoclinic pyribole; and extended displacive faults and rotational faults that apparently result from physical deforma- tion. Two types of talc have 6"sa disfin8uished by their orientation relationships with the pyri- boles.In "coherent talc," the relationship is D,.llDo'ar"llco, cnlla,,. Structural considerations and high-resolutionimages suggest that this type of talc possesses20 stacking.In "(210)-talc" there are no rigid orientation relationships with the intergrown pyribole, but the sheetsof the talc tend to be parallel or subparallelto (210) ofthe pyribole. This type oftalc displaysex- treme stackingdisorder. Textural evidence suggeststhat the wide-chain silicates and fibrous talc at Chester formed by retrograde hydration reaction of anthophyllite. This conclusion is supported by the micro- structures observed with electron microscopy. These microstructures suggestthat there were several mechanismsfor 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 sequenceanthophyllite -r disordered chain silicate --+ chesterite ---) more disordered chain silicate --+ jimthomFsonite + talc or dis- ordered chain silicate with very wide chains. The disordering steps of this sequenceappar- ently occurred by the nucleation and growth of wide-chain zippers. When a region of crystal achieveda chemical compositionclose to that of one of the ordered phases,reordering oc- curred by the passageof en echelontermination faults through the structure in the a direction. Much of the chemical transport necessaryfor these reactions may have taken place by "ultra- fast" difusion along large structural tunnels associatedwith zipper terminations. 0003-0Mx/80/0708-0s99$02.00 VEBLEN AND BUSECK: MICROSTRUCTURES IN BIOPYRIBOLES Introduction reactions.The final sectionconsiders mechanisms for It has long been known that biopyriboles may un- the chemical transport that is an integral part of these dergo oriented hydration reactions that can trans- reaction processes. form a pyroxene into an amphibole, or an amphibole This study is part of a broader investigationof bio- into a mica, for example. The former type of reac- pyribole reactions. Preliminary results on pyroxene tion, which has been called uralitization, was clearly hydration reactionsindicate that, in some cases,the recognized and described by Goldschmidt (1911, p. mechanisms are exactly analogous to the mecha- 345). Veblen and Burnham (1978a,b) have shown nismsdescribed here (Veblen and Buseck,1978). Our that hydration reaction of amphibole to talc can be results are thus not restricted to a single locality or prove accompanied by the production of additional or- even a single mineral group, but may instead dered wide-chain silicates, such as chesterite and jim- to be typical of all pyriboles undergoing hydration thompsonite, as well as structurally disordered bio- reaction. Furthermore, microstructures similar to pyribole having a variety of silicate chain widths. someof thosedescribed here are observedin octahe- High-resolution transmission electron microscopy dral chain structures, such as manganese oxides (HRTEM) has been used to elucidate the structure and (Turner and Buseck, 1979),suggesting even broader spatial distribution of ordered and disordered mate- apptcability of the presentwork. rial resulting from the anthophyllite-talc reaction in Image interpretation and experimental techniques an ultramafic body at Chester, Vermont (Veblen er al., 1977; Veblen and Buseck, 1979a). The present Veblen and Buseck(1979a) showed that ferromag- HRTEM study looks beyond the question of chain- nesian biopyribolesproduce c-axis snrEu imagesin width order and disorder and instead considers the which dark areascorrespond to the positionsof the I- rich variety of structural defects found in the Chester beams in the structure. Correlation of such images biopyriboles. These defects, which were apparently with difraction patterns and dynamical diffraction created during a sequence of metamorphic reactions and imaging calculations showed that this relation- and deformations in the Chester Dome, can be used ship holds for a wide range of instrumental condi- to understand the mechanisms of the hydration reac- tions and crystal thicknesses;the widths of pyribole tions. Similar textural methods g5ing crystal defects I-beamscan be recognizedin such images.This rela- have been used to explore reaction mechanisms in tionship is further substantiated by defect calcu- oxide systems (Obayashi and Anderson,1976). lations presentedhere. Veblen et al. (1977) and Jef- The structural interpretation of pyribole HRTEM ferson et al. (1978) showed that pyriboles also images is now relatively straightforward. The inter- produce interpretable a-axis images. Experimental pretation of defects in the Chester biopyriboles in technique and image interpretation for the present terms of a reaction rrodel may, however, be open to work thus follow methods describedpreviously. We discussion. Although there is clear textural and mi- assumethat the reader who wishesto understandthe crostructural evidence that much of the wide-chain figures in the present paper will be familiar with silicate and fibrous talc was formed by hydration re- those in the earlier work and with the l-beam repre- action from amphibole, there is no rigorous proof sentation of pyribole structures(Veblen et al., 1977; that all of it formed in this way; some of the material Veblen and Buseck, 1979a). with triple and wider chains was possibly produced The c-axis inages in this paper were produced by primary growth. As a result of this ambiguity, we from ion-milled specimens,whereas a-axis images have chosen to present our observations first, with are from samplesthat were crushed under acetone very little interpretation, and then to interpret the ob- and suspendedon holey carbon grids. All specimens servations in terms of a reaction model. The first half are from a block of blackwall reaction zone from a of this paper is thus a catalog of microstructures ob- metamorphosedultramafic body near Chester,Ver- served in the Chester pyriboles. These include several mont, as describedby Veblen and Burnham (1978a). types of Wadsley defect ("zipper") terminations, stacking faults, partial dislocations, column defects, Catalog of pyribole microstructures chain terminations, low-angle grain boundaries, "Zippers" monoclinic lamellae, intergrowths with talc, and and zipperterminations phase boundaries. Evidence for the formation of the Silicatechains (and hencel-beams) of the "wrong" Chester pyriboles by reaction from amphibole is then width are corlmon in the Chesterbiopyriboles; they summarized, and the above microstructures are used may occur as isolated faults or as broad areashaving to delineate possible mechanisms for the hydration completely disordered chain widths. Strictly speak- VEBLEN AND BUSECK: MICROSTRUCTURES IN BIOPYRIBOLES 601 ing, these chain-width enors are Wadsley defects imental image it appears that the structure around (Wadsley and Anderson, 1970; Chisholm, 1973) or the channel has relaxed slightly into the defect. The polysomatic defects (Thompson, 1978),but for the calculated image, however, shows the same effect in '.zip- sakeof sirnplicity, we will refer to them here as its lowermost sextuple-chain I-beam, demonstrating pers," which they resemblein c-axisorientation. The that even a completely