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Sedimentary Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
The general term for replacement processes, including that outlined above, is metasomatism (= change of substance, Greek); original minerals are changed atom by atom into new mineral substances by the agency of percolating solutions, but the outlines of original structures in the rock are frequently preserved. The dolomitization of a limestone thus involves the replacement of part of the calcium by magnesium. Another metasomatic change which takes place in limestones is the replacement of calcium carbonate by silica; in this way silicified limestones and cherts are formed, as in parts of the Carboniferous Limestone. Silica from organisms such as sponges and radiolaria is dissolved by water containing potassium carbonate, and re-deposited as chert, which is a form of cryptocrystalline silica. The Hythe Beds of the Lower Greensand in Kent have beds of chert of considerable thickness. Replacement of calcite by siderite (FeCO3) is another important metasomatic change; it is seen in some Jurassic limestones such as the Cornbrash of Yorkshire and the Middle Lias marlstone of the Midlands. The Cleveland Ironstone of Yorkshire, which is oolitic, is believed to have originated in this way.
Fluid inclusions in ore-bearing dolomite, Silesian–Cracow area, Poland
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
The ore-bearing dolomites from the Silesian-Cracow area bear fluid inclusions which indicate low-temperature metasomatic origin of the rock (Fig. 3). The concentrations of the metasomatising fluid increased gradually to 20% (Fig. 4). The fluids had in part different features than those that caused crystallisation of ores in the studied area. Only the two latest types of inclusions [4] and [5] are similar to two inclusion groups, found in ores and attributed to connate and formation waters (Kozłowski, 1995). Thus, the parent fluids of the ore-bearing dolomites were of different origins and appeared earlier, than the ore-forming fluids (Fig. 5). They were similar to meteoric waters and formation brines, but the third component, what was apparently present in the parent fluids of the sulphide Zn-Pb ores, i.e. ascending waters of relatively high temperature, was absent in the dolomitising solutions. The variations of the chemical composition of the dolomitising fluids resulted from the dolomitisation reaction, i/e. consumption of magnesium and release of calcium. No differences were found in samples from the Olkusz, Chrzanów and Bytom districts.
Hyperspectral data as a proxy for porosity estimation of carbonate rocks
Published in Australian Journal of Earth Sciences, 2022
L. S. Kupssinskü, T. T. Guimarães, M. d. B. Cardoso, L. Bachi, D. Zanotta, I. Estilon de Souza, A. X. Falcão, R. Q. Velloso, C. L. Cazarin, M. R. Veronez, L. Gonzaga
Dolomitisation, which occurs in both study areas, changes calcite to dolomite when exposed to waters rich in magnesium and involves large-scale recrystallisation with the potential to increase or decrease porosity in carbonate rocks. During the first stages of dolomitisation, mineral replacement generates an increase in interparticle porosity, since the volume of dolomite is smaller than the volume of calcite. However, the mineral replacement and dolomite growth are a continuous process that leads to a reduction in porosity, especially in the peripheral zones of the carbonate platform. The primary porosity can be reduced by growth of dolomite crystals, and dolomite cement can fill the intraparticular pores and eventually reduce porosity (Purser et al., 2009).