Calcite grain boundaries

Uranium is distributed uniformly within individual growth layers, but varies greatly in concentration between successive layers (A, B, and C). The distribution of tracks suggests that U is distributed homogeneously in single calcite crystals, and is not concentrated at grain boundaries or in inclusions. 

It can be demonstrated that grain boundaries or microcracks in sediments are preferred environments of cement nucleation. Consider the environment between two grains of carbonate separated by a void area of radius r (Figure 7.17 ) the energy (AFcav) required for the formation of calcite cement domains in such a cavity of height h and radius r is ... 

Figure 8.37. Lattice-fringe image showing a high-angle grain boundary in a bicrystal of wet synthetic calcite (From Hay and Evans 1988.)...

Figure 17 Oxygen isotope composition versus distance for unaltered calcite grains from the Hida metamorphic belt, Japan. Each set of symbols reflects a separate zoning profile measurement. Data represented by circles have been corrected for an estimated 30° dip of the grain boundary. These profiles are well described by diffusional exchange (fitted curves) (source Graham et al., 1998).

Farver J. R. and Yund R. A. (1996) Volume and grain boundary diffusion of calcium in natural and hot-pressed calcite aggregates. Contribut. Mineral. Petrol. 123, 77-91. 

Farver JR, Yund RA (1995b) Grain boundary diffusion of oxygen, potassium and calcium in natural and hot-pressed feldspar aggregates. Contrib Mineral Petrol 118 340-355 Farver JR, Yund RA (1998) Oxygen grain boundary diffusion in natural and hot-pressed calcite aggregates. Earth Planet Sci Lett 161 189-200... 

Farver JR, Yund RA (1998) Oxygen grain boundary diffusion in natural and hot-pressed calcite aggregates. Earth Planet Sci Letters 161 189-200... 

Corrales LR (1999) Dissociative model of water clusters. J Chemical Physics 110 9071-9080 Curtiss LA, Halley JW, Hautman J, Rahman A (1987) Nonadditivity of ab-initio pair potentials for molecular dynamics of multivalent transition metal ions in water. J Chem Phys 86 2319-2327 de Leeuw NH, Parker SC, Catlow CRA, Price GD (2000) Proton-containing defects at forsterite (010) tilt grain boundaries and stepped surfaces. Am Min 85 1143-1154 de Leeuw NH, Parker SC (1998) Surface stracture and morphology of calcium carbonate polymorphs calcite, aragonite, andvaterite An atomistic approach. JPhys ChemB 102 2914-2922... 

Bridging between crystals with a chemical composition differing from that of the solution in which they are immersed, such as quartz-quartz or calcite-calcite, should apparently involve the formation of two grain boundaries, that is, o = 2(03 - Oj) (Figure 6.7c). Such bridging is thermodynamically less favorable (and hence less probable) than bridging under conditions when the chemical composition of the solution is similar to that of one or both crystals. Therefore, in this case, the values of W, are low. 

B) Boundary between calcite-cemented and uncemented horizons in Avalon Sandstone, showing no evidence of dissolution. Straight crystal faces of the poikilotopic calcite (left) at the boundary (centre) indicate the presence of a cementation front, rather than a dissolution front. Note that framework grains in the porous zone are coated with thin clay rims, which are absent in the cemented zone. The former is also slightly more compacted than the latter. Same locality as... 

Influence of "Early" Cement. The presence of calcite, dolomite and anhydrite in the form of early cement strongly hinders pressure solution and thereby contributes to the preservation of intergranular space. This observation may also be made for early quartzose cement. The latter contributes to the formation of a solid framework in the sandstones which obstructs their later compaction by equalizing tension along the boundaries of the quartz grains of the framework. 

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