Insights into the growth morphology of calcite cement

Abstract

The classic work on the morphology of limestone calcite cements done in the 1960s is extended here by utilising growth zones to reconstruct the growth of cement crystals. Only cement composed of fitted polyhedral monocrystals that form by passive crystallisation of calcite on the walls of liquid-filled, static pores and fissures is considered. Cement can either be initiated by (1) nucleation, when new crystals start but are not attached to their substrate, or (2) seeding, when new crystals are seamlessly connected to and influenced by substrate crystals. After seeding, epitaxial cement growth starts with many sub-crystals that coalesce distally, followed by layered mantle growth. Junctions between three intercrystalline boundaries in cement aggregates with one interfacial angel = 180° are of two types: the first, enfacial junctions are caused by a pause in the growth of one crystal and the second is caused by movement of all boundaries due to dissolution of adjacent calcite. Growth zone offsetting at some intercrystalline boundaries is caused by dissolution of calcite at boundaries when permeability values are low. The same width to height ratio of mature aggregate crystals is predicted from the shape of the crystal's growth surfaces; dogtooth calcite forms columnar and nail-head calcite forms tabular-shaped crystals. Seeding on different sized crystals causes variations in epitaxial growth rate with faster growth on large crystals resulting in a disorganised cement fabric; the variation in epitaxial growth rate is perpetuated into mantle growth. Echinoderm syntaxial crystals dominate many pore cements due to the large size of their seed ossicles, at the same time, syntaxial crystals form on relatively tiny seeds. Texturally mature crystal aggregates with isopachous fabric are initiated from three different substrate to cement arrangements. Calcite cement zones preserve their original positions allowing the investigation of cement's growth and chemical history.