POLYSOMATISM, POLYTYPISM, DEFECT MICROSTRUCTURES, AND REACTION-MECHANISMS IN REGULARLY AND RANDOMLY INTERSTRATIFIED SERPENTINE AND CHLORITE

High-resolution (HRTEM) and analytical electron (AEM) microscopic evid ence for a polysomatic series based on regular interstratifications of serpentine (amesite) and chlorite (clinochlore) are reported from an altered skarn in Irian Jaya. The assemblage includes regular interstra tifications of one clinochlore and two (2:1; three structural variants ), three (3:1), and four (4:1) amesite composition 1:1 layers as well as randomly interstratified serpentine and chlorite. The order of abun dance of regularly interstratifted minerals is 1:1 > 2:1 > 4:1 > 3:1. Atomic-resolution images, image simulations, and comparison between ca lculated and observed diffracted intensities verify the proposed 1:1 a nd 2: 1 structures and reveal details of their defect microstructures. AEM data show that compositions are linear combinations of the associ ated amesite and clinochlore. The 1:1, 2:1, 3:1, and 4:1 minerals occu r both as discrete sub-micron crystals and as domains within serpentin e or chlorite. Some crystals of the 2:1 phase were sufficiently large for study by X-ray precession and powder methods. Crystals of the regu larly interstratified 2:1, 3:1, and 4:1 phases are usually bent. High- resolution images reveal that, within polygonal segments, the layers c ommonly exhibit a few degrees of curvature with segments separated by antigorite-type offsets. Deformed chlorite crystals are probably repla ced by interstratified minerals during an aluminum metasomatic event. Al may have been deposited from sulfuric acid-rich solutions when they interacted with calcite and dolomite to form the anhydrite-rich coron a around the phyllosilicate-rich region of the core. The interstratifi ed chlorite (clinochlore composition) suggests aluminum addition by se lective conversion of a sub-set of the chlorite layers to amesite. Def ect microstructures suggest that crystals of regularly interstratified material grew by direct structural modification of preexisting chlori te. Regular interstratifications may form in response to thermally con trolled limits on Al solubility in chlorite and heterogeneities in the distribution of Al-rich solutions during metasomatism. Regularly inte rstratified minerals coexist with randomly interstratified serpentine/ chlorite, chrysotile, antigorite, lizardite, and several amesite and c hlorite polytypes. Tentative chlorite and amesite identifications incl ude one-layer (b = 97-degrees, probably IIbb, one-layer (b = 90, possi bly Ibb), two-, and three-layer chlorites, and H-1(2) (but possibly IM or 1T), rhombohedral (3R or 6R), and twelve-layer (Tc; non standard) serpentine polytypes. The complex phyllosilicates attest to rampant ch emical and structural disequilibrium.