MORPHOLOGY, RELATIONSHIP, AND ORIGIN OF FIBER AND DENDRITE CALCITE CRYSTALS

Fiber and dendrite calcite crystals, formed by abnormal growth conditi ons, are minor but important components of many limestones that have u ndergone vadose meteoric diagenesis. Fiber crystals, which have a leng th:width ratio of greater than 6:1, are divided into hexagonal fibers, rectangular fibers, composite fibers, and rhomb chains according to t heir crystal form. Dendrite crystals are formed of a main stem, primar y branches, and secondary branches. Random and tangential fiber crysta ls refer to masses of fibers that have a random distribution or tangen tially coat a substrate, respectively. Such crystals may be tightly or loosely packed. The shape of fiber and dendrite crystals can be subst antially modified by destructive or constructive diagenetic processes. Destructive modification includes physical breakdown, dissolution, or micritization. Constructive modification, which usually involves epit axial growth around the original fiber or dendrite crystal, may procee d to the point where the original crystals are completely disguised. I n extreme cases fiber crystals can be cemented together to form lattic es that look like dendrite crystals. Fiber and dendrite calcite crysta ls 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 ex amples where both are present together in nature. Such crystals form t hrough biogenically and abiogenically mediated processes. The common a ssociation of these crystals with plant roots or microorganisms sugges ts that the organisms can produce conditions suitable for growth of th e crystals.