Self-organizational origin of agates: Banding, fiber twisting, composition, and dynamic crystallization model
Agates display associated repetitive textures and composition: alternating layers consisting of twisted and non-twisted chalcedony fibers and correspondingly high and low Al concentration, banding of water inclusions, chevron interference, and micrometer-scale concentric bands with alternating high and low OH content. This association suggests a self-organizational crystallization, a process in which systematically repetitive textures and composition are produced by the very dynamics of reaction-transport interactions, not by periodically changing outside causes. An isothermal, quantitative, dynamic model is proposed here based on the autocatalytic reaction, Initial Medium Al = Quartz + Al. Al 3+ is released by the reaction and is assumed to act as a catalyst for it. The model consists of equations taking account of mass continuity, diffusion, advection, and quartz crystallization. A linear instability analysis of the equations indicates that, for self-organizational behavior to be possible, agate crystallization must take place from an initial medium having silica concentration ≥ 1 g SiO 2/cm 3. This huge threshold alone strongly points to closed-system crystallization from lumps of gel or amorphous silica, but not from aqueous solutions. Numerical solutions of the full nonlinear system of equations for appropriate parameter values indeed yield seesaw (and out-of-phase) profiles of silica and Al concentration at the crystallization front. Chalcedony fibers that grow under high Al incorporate considerable Al for Si. Because the Al taken up becomes ordered and because Al 3+ is larger than Si 4+, this substitution causes the fibers to grow twisted. But fibers that grow under low Al at the front incorporate no or (even for quartz) little Al, and do not become twisted. The model correctly accounts in an integrated way for many textural and compositional properties of agates: generation and spacing of repetitive banding; formation of alternating twisted-fiber and nontwistedfiber chalcedony layers and their corresponding high and low Al content; periodic trapping of fluid inclusions at moments of very high quartz growth rate; general inward sequence of a repetitively banded region, a non-banded region, and a central void; a generally decreasing δ 18O profile from the outside inward with sharp increases at predicted steam flashing. Each chalcedony layer is predicted to crystallize in tens of hours.