A PROPOSED MECHANISM FOR THE GROWTH OF CHALCEDONY

The structural disparities that distinguish chalcedony from macrocryst alline quartz suggest that different crystallization mechanisms are op erative during the growth of these two forms of silica. Although the p aragenesis of chalcedony has provoked marked disagreement among resear chers, a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relativel y low temperatures (< 100-degrees-C). These conditions for deposition suggest a model for chalcedony crystallization that involves the assem bly of short-chain linear polymers via bridging silica monomers. This assembly occurs through a spiral growth mechanism activated by a screw dislocation with b = n/2 [110], where n is an integer. The proposed m odel can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale, such as: (1) the directio n of fiber elongation along [110] rather than [001]; (2) the periodic twisting of chalcedony fibers about [110]; (3) the high density of Bra zil twin composition planes; (4) the common intergrowth of moganite wi thin chalcedony.