Graphical modelling of crystal aggregates and its relevance to cement diagnosis

Abstract
The identification of carbonate cements in rock sections is accomplished by means of textural criteria. Some of these criteria, relating to crystal fabric, may be modelled graphically in crystal growth diagrams. Existing crystal growth diagrams are oversimplified. Three new crystal growth diagrams are presented for calcite {1011}, {4041} and {0112} rhombohedra The crystals are envisaged as growing by zones of equal width from an inert substrate. The patterns that growth zones create on cuts through individual crystals are used as templates to produce the crystal growth diagrams. 151 crystals seeded on the substrate, on both sides of the crystal growth diagram, are considered to grow at identical rates and impinge on one another. The three crystal growth diagrams illustrate features of crystal initiation and shape in two dimensions not found in previously published diagrams. The rate of maturation of the crystal fabric away from the substrate is shown to depend partly on crystallographic form. Optical elongation is also shown to be related to crystallographic form. The enfacial junction, a special type of triple junction, is shown to be absent from the crystal growth diagrams. The diagrams presented are constrained. Changes in (i) the direction of crystal cutting, (ii) crystallographic form during growth, (iii) the number of crystallographic forms at one time, (iv) the positioning of nuclei, (v) shape of the substrate, and (vi) the change to an active substrate where epitaxy occurs, would all change the character of the crystal growth diagram. The technique of using a list of textural criteria for identifying cements is rejected. Such lists may be used to identify the morphological type of crystal aggregate. After examination of other properties, the decision as to whether that type of aggregate is cement or not can be made. The recognition of impingement growth is made easy if all stages of maturation are present. Where insufficient space is available for mature growth to develop, recognition must rely on features other than textural. A calcite aggregate developed in a Permian bryozoan biolithite is used to illustrate immature impingement growth. This aggregate is identified as of impingement type principally from the arrangement of growth zonation shown in each crystal by cathodoluminescence. The enfacial junction, often regarded as the least equivocal criterion for the recognition of cements, is absent from the crystal growth diagrams and the Permian biolithite. The origin of enfacial junctions in a Lower Carboniferous spar is thought to be due either to unusual sporadic growth inhibition or, more likely, to intercrystalline boundary migration due to dissolution. Enfacial junctions are common in ancient spars: if generated by intercrystalline dissolution considerable quantities of CaCO 3 have been liberated. Enfacial junctions also develop at the junction of two generations of crystals not exhibiting epitaxy.

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