Chain-width order and disorder in biopyriboles

Abstract

High-resolution c-axis images of the biopyribole minerals anthophyllite [(Mg,Fe)' Si8Orr(OH)rl, jimthompsonite [(Mg,Fe),oSi,rOrr(OH)o], and chesterite [(Mg'Fe),rSiro O'4(OH)61 have been produced in the transmission electron microscope. Interpretation of the images has been confirmed by dynamical electron di-ffraction and imaging calculations. In material from Chester, Vermont, the chain width of areas corresponding to more than one million amphibole chains in the b direction has been imaged and examined. In addition to perfectly ordered phases, parts of some crystals exhibit chain-width disorder ranging from minor faulting of otherwise ordered material to extreme disorder in the chain sequence. Individual silicate chains with widths corresponding to as many as 333 pyroxene chains have been observed. Several new ordered mixed-chain pyriboles have been found to occur in limited but statistically signifrcant amounts. These new ordered structures have primitive unitcell chain sequences (2233), (233), (232233), (222333), (2332323), (2333), (433323), (2234), and (43332343332423). A method for distinguishing statistically-significant ordered structures from those that arise simply from random combination of structural elements is described. If the minerals chesterite and jimthompsonite are stable, their stability ranges are probably between those of anthophyllite and talc. The ordered minerals may, however, be produced metastably during reaction, as the result of constraints imposed by the reaction mechanisms. The occurrence of chesterite appears to be controlled by the chemical potentials of Fe, Ca, and Mg. The newly observed long-range ordered structures are probably the result of some non-random reaction or growth mechanism, rather than being thermodynamically stabilized.