Natural calcite in cathodoluminescence: crystal growth during diagenesis

Abstract

Some of the largest crystals found as pore-filling cements in ancient limestones are those which develop as single-crystal (syntaxial) overgrowths on crinoid grains. Although originally porous and polycrystalline^1,2, such grains normally behave as monocrystalline substrates during diagenesis, facilitating the relatively rapid growth of optically contiguous calcite haloes. The crystal form and growth morphology of pore-fill cements can yield important information about the diagenetic environment¹, but growth features cannot easily be detected with the light microscope. Chemical stains have been used to study internal growth zonation^3,4 but cathodoluminescence⁵ has proved a more satisfactory technique^6,7 and our own studies of calcite have revealed a variety of novel growth phenomena. We distinguish four styles of syntaxial overgrowth, the morphologies of which appear to depend on three main factors: grain surface conditions, surrounding pore conditions and the nature of the diagenetic environment.