Abstract

Fiber and dendrite calcite crystals, formed by abnormal growth conditions, are minor but important components of many limestones that have undergone vadose meteoric diagenesis. Fiber crystals, which have a length: width ratio of greater than 6:1, are divided into hexagonal fibers, rectangular fibers, composite fibers, and rhomb chains according to their crystal form. Dendrite crystals are formed of a main stem, primary branches, and secondary branches. Random and tangential fiber crystals refer to masses of fibers that have a random distribution or tangentially coat a substrate, respectively. Such crystals may be tightly or loosely packed. The shape of fiber and dendrite crystals can be substantially modified by destructive or constructive diagenetic processes. Destructive modification includes physical breakdown, dissolution, or micritization. Constructive modification, which usually involves epitaxial growth around the original fiber or dendrite crystal, may proceed to the point where the original crystals are completely disguised. In extreme cases fiber crystals can be cemented together to form lattices that look like dendrite crystals. Fiber and dendrite calcite crystals generally form from fluids that are supersaturated with respect to calcium carbonate. Although experimental data suggest that both types of crystal can form from the same parent fluid, there are only rare examples where both are present together in nature. Such crystals form through biogenically and abiogenically mediated processes. The common association of these crystals with plant roots or microorganisms suggests that the organisms can produce conditions suitable for growth of the crystals.

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