Aragonite analysis

Although equilibrium was not established, it was more closely approached in the KCl-KBr-H20 system than in carbonate systems. For example, in a similar analysis of the strontianite-aragonite solid solution system (4 ), it was found that the experimental distribution coefficient for Sr substitution from seawater into aragonite is 12 times larger than the expected equilibrium value. Most of the distribution coefficients for the KCl-KBr-H20 system are within a factor of two of the equilibrium value, but clearly not at equilibrium. Considerable caution should be exercised before reaching the conclusion that equilibrium is established at relatively low temperatures in other solid solution-aqueous solution systems. 

This method of analysis is particularly valuable when chemical methods are inadequate or inapplicable. For instance, for complex mixtures where the different elements or ions may be associated in many different ways, all compatible with the analytical figures or for mixtures of polymorphous forms of the same substance, such as the three crystalline forms of CaC03 (calcite, aragonite, and vaterite) or the three crystalline forms of FeO(OH) (goethite, lepidocrocite, and P FeO(OH)—see Bunn, 1941)—mixtures for which chemical analytical methods are irrelevant. 

Prior to analysis on the IRMS, powdered aragonite samples were gently heated at 75°C in vacuo for 30 minutes to remove adsorbed water and... 

Heating experiments on Nautilus shell material (see page 10) indicates a retardation in the rates of thermal decomposition of mineral and organic phase. This has been attributed to the development of a topochemical boundary at the contact protein-mineral. X-ray diffraction analysis gives no evidence for structural alteration of the original aragonite during the thermal treatment up to 200 °C. 

On a relative basis, i.e. residues per 1000, there is virtually no one species like the other. In contrast, different shell samples from the same species and obtained from the same natural habitat yield identical amino acid patterns. It is of interest that (1) the structure of carbonates (aragonite-calcite-vaterite), (2) the content in trace elements, and (3) the stable isotope distribution are markedly effected by fluctuations in salinity, water temperature, Eh/pH conditions, and some anthropogenic factors. The same environmental parameters determine to a certain degree the chemical composition of the shell organic matrix. This feature suggests a cause-effect relationship between mineralogy and organic chemistry of a shell. In the final analysis, however, it is simply a reflection of the environmentally-controlled dynamics of the cell. 

The applicability of scanning Auger spectroscopy to the analysis of carbonate mineral surface reactions was demonstrated by Mucci and Morse (1985), who carried out an investigation of Mg2+ adsorption on calcite, aragonite, magnesite, and dolomite surfaces from synthetic seawater at two saturation states. Results are summarized in Table 2.5. 

The analysis of surface seawater shows it to be moderately supersaturated with respect to both calcite (SI - 0.70) and disordered dolomite (Sl,i = 0.88) and more supersaturated with respect to ordered dolomite (SI = 1.65). It is also supersaturated with respect to magnesite (5/ , = 0.76 assuming K, = 10 ), and aragonite (5/ = 0.56). Equilibration of seawater with re.spect to ordered dolomite, magnesite, and pure calcite is kinetically inhibited (high Mg " may inhibit calcite nucleation in seawater), although aragonite does readily precipitate from seawater. 

X-ray diffraction analysis of the products of the benefici-ation of Dow-Colony shale is shown in Table I. It is clear that the oleophilic extract (pellets) retains or concentrates the calcium and magnesium carbonates (dolomite, calcite, aragonite), particularly the dolomite. The hydrophilic gangue consists mainly of feldspar and quartz. Since silica, silicates and Ca/Ca-Mg carbonates, in a clean condition, are water wettable... 

ALLISON, N. 1996. Comparative determinations of trace and minor elements in coral aragonite by ion microprobe analysis, with preUminary results from Phuket, southern Thailand. Geochimica et Cosmochimica Acta, 60, 3457-3470. 

The vibrational spectra of calcite and aragonite crystals are markedly different, although both have the same composition (Sec. 2.4). This result can be explained if we consider the difference in site symmetry of the COl ion between these crystals. According to X-ray analysis, the space group of calcite is and Z is 2 Appendix X... 

Figure 18 shows the fluctuation in interpreted chemocline depth of Waldsea Lake over the past 2000 years based on the mean crystal size of aragonite crystals in two cores in the basin. The Core A, the same core on which the detailed aragonite shape analysis was done, is located near the deepest (14 m) part of the basin. Core B is located about a kilometer... 

The isotopic analysis of calcite and aragonite can be accomplished by liberating CO2 in a reaction with 104% phosphoric acid in the classic method described by McCrea (1950). 

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