Needle-fiber calcite; a critical review and a proposed classification

Abstract From a review of the recent literature and our own observations, the different morphologies of needle-fiber calcite are classified. They can be characterized as monocrystalline rods (with or without serrated edges) or as polycrystalline chains. The simplest monocrystalline habit of needle-fiber calcite is the individual micro-rod (M). Larger monocrystalline rods (MA) exist in multiples of two. More complex forms are found as paired monocrystalline rods with serrated plates along their length (MB). This group includes three habits, which can be distinguished by the number of plates and their orientation relative to the rod. Finally, there is a monocrystalline form (MAB), intermediate between the categories MA and MB, which shows the transition from the simple coupled rods to rods with serrated plates. Although polycrystalline needles (P) have rarely been reported in the literature, three types of morphologies have been identified. Each type of crystal has a specific origin. The M micro-rods are physicochemical nuclei or calcified bacteria. They do not seem to evolve towards larger or more complex morphologies. The MA rods are related to biomineralization inside the fungal sheaths. The MB rods postdate liberation of MA rods from mycelian bundles. The serrated plates are secondary syntactic crystallizations of physicochemical origin, whose growth can be inhibited for various reasons. The origin of P chains is less well understood, but they seem to be physicochemical crystallizations related to evaporation and desiccation. Therefore, these three groups--M, MA-MB, and P--have similar morphologies but do not seem to be related genetically. The only known link is between the MA and MB rods. There is no evolutionary sequence of needle-fiber calcite habits but a convergence of forms between crystals whose origins are different and independent. Structures described in the literature as "convoluted", "random", "cutane", and "nest-like" are the result of either evolution of the mycelian bundles or movement of material and organisms in the soil. In the case of M micro-rods, the microfabric is related to the random precipitation around nuclei in pores filled with a highly supersaturated solution. Initially, the P chains also precipitate without preferential organization, and later on, the growth seems to preferentially constitute chains, although the mechanisms are still not understood. Moreover, needle-fiber calcites do not indicate a particular climatic environment.