Caves, carbonate deposition

C06-0134. The reaction that forms calcium carbonate deposits in caves is as follows ... 

Caves are important environments of carbonate deposition but, although there is some evidence for biogenic calcification vrithin them (Friedman, 1955 Pochon et al., 1964 Schneider, 1977), it is probable that, here, inorganic carbonate deposition is the major factor. 

Travertine 1) Any finely crystalline massive concretionary limestone formed by surface or groundwater evaporation. 2) Any flowstone deposit, hence the usage travertine implying stalagmitic flowstone. 3) Any carbonate deposit formed within a cave or karst system by secondary precipitation. 4) Massive, hard, dense carbonate deposits formed in association with open-air springs, streams, lakes, or marshes that precipitate carbonate. 

When the solution reaches a cave, carbon dioxide slowly escapes from the solution, and the subsequent shift in the equilibrium causes CaC03 to be precipitated. Deposits of CaC03 produce the attractive cave formations. 

The results for all sites are given in Table 5.1, and are best considered by dividing sites into three groups according to isotopic nature of the matrix (i) sites with most isotopically emiched matrix carbonates (Die Kelders and Swartkrans), (ii) sites with rather less enriched carbonates (Klasies River Mouth and Makapansgat), and finally (iii) a site with depleted deposit values (Border Cave). This is summarized in Fig. 5.5. The division also fortuitously provides a range of age depths in two categories. As indicated in Table 5.1, many of these data have been published elsewhere, but the purpose for which they are considered in combination here has not been previously attempted. 

These stalactites in a cave in the Great Basin National Park in Nevada are deposits of calcium carbonate formed as drops of water have evaporated. Calcium forms a small, highly charged cation and consequently gives rise to rigid structures. 

Table II shows the quantitative XPS results for all of the samples studied. Not unexpectedly, the dolomite substrate showed the least amount of carbon of all the samples. Two samples, Sample 2 (the shiny coating with little substrate) and Sample 3, had high levels of nitrogen, significantly more than would be expected for a humic or fiilvic acid. The atomic compositions of the other coating samples compared well with experimental and theoretical C, N, and O contents for humic acids. The standard humic acid from Alfa Aesar contained a significant amount of K as an impurity. The origin of this particular humic acid is not known, even to Alfa Aesar. Traces of Ca, Cl, Fe, and Si are likely from dissolved salts that are deposited at the cave surface.

Calcium carbonate (CaC03) occurs naturally as calcite (density 2.7), a widely distributed mineral. Calcite is a common constituent of sedimentary rocks, as a vein mineral, and as deposits from hot springs and in caves as stalactites and stalagmites. Calcite is white or colorless through shades of gray, red, yellow, green, blue, violet, brown, or even black when charged with impurities streaked, white transparent to opaque. It may occasionally show phosphorescence or fluorescence. 

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