Electron microscope study of microtexture and grain surfaces in limestones

Electron micrographs of directcarbon replicas having magnifications of 5400 to 23, 500 times reveal in detail the grain size, shape, and packing of the particles comprising very fine-grained limestones. At these magnifications, however, only portions of a few grains in mediumor coarsegrained limestones can be observed. Grain contacts are mostly curvilinear in very fine-grained limestones but are more frequently straight in coarse-grained limestones. The width of the grain contacts varies. Breakage of limestone by tension produces fractures that follow grain contacts in some places and cross grains in others. Even very fine-grained limestones (micritic limestones) fracture across many grains, and thus cleavage fracturing characterizes tensile breakage in limestones. Shear fracture surfaces are characterized by slip and/or twin lamellae, partial grain cracks, surface grooves on large grains, and a fine dust on some grain surfaces, thought to have resulted from abrasion during fracturing. Some cleavage fracturing also occurs on shear surfaces. Eighteen electron micrographs illustrate these and other features.