Hydrothermal exchange

Roy, D. M. Linnehan, S. Hydroxyapatite Formed from Coral Skeleton Carbonate by Hydrothermal Exchange. Nature (London) 1974,247, 220. 

The best-studied example of a hydrothermal system associated with a gabbro is the Skaergaard intrusion (Taylor and Forester 1979 Norton and Taylor 1979). The latter authors carried out a computer simulation of the Skaergaard hydrothermal system and fonnd a good match between calculated and measured 8 0-values. They further demonstrated that most of the subsolidus hydrothermal exchange took place at very high temperatnres (400-800° C), which is compatible with the general... 

Valley J. W. and Graham C. M. (1996) Ion microprobe analysis of oxygen isotope ratios in quartz from Skye granite healed micro-cracks, fluid flow, and hydrothermal exchange. Contrib. Mineral. Petrol. 124, 225-234. 

Synthesis of solid solutions of hydroxyflnorapatite. Submitted to J Mater Chem Roy D, Linnehan SK (1974) Hydroxylapatite formed from coral skeletal carbonate by hydrothermal exchange. Nature 247 220-222... 

Coral is a natural substance made by marine invertebrates. According to Holmes et al. [1984], the marine invertebrates live in the limestone exostructure, or coral. The porous structure of the coral is unique for each species of marine invertebrate [Holmes et al., 1984]. Corals for use as bone implants are selected on the basis of structural similarity to bone [Holmes et al., 1984]. Coral provides an excellent structure for the ingrowth of bone, and the main component, calcium carbonate, is gradually resorbed by the body [Khavari and Bajpai, 1993]. Corals can also be converted to hydroxyapatite by a hydrothermal exchange process. Interpore 200, a coral hydroxyapatite, resembles cancellous bone [Sartoriset al., 1986]. Both pure coral (Biocoral) and coral transformed to hydroxyapatite are currently used to repair traumatized bone, replace diseased bone, and correct various bone defects. 

The natural-template method was developed in 1974. It can produce porous HA powders. A suitable template was found to be the calcium carbonate (CaC03) skeleton of reef-building corals, such as those found in the South Pacific. The reaction to produce HA involves a hydrothermal exchange reaction of carbonate groups with phosphate groups, which can occur via the following chemical reaction ... 

Fig. 6. In a binary electricity generation plant, the hydrothermal water from the weU, A, is passed through a heat exchanger, B, where its thermal energy is transferred to a second, more volatile working fluid. The second fluid is vaporized and deflvered to a turbine, D. After exiting the turbine the spent working fluid is cooled and recondensed in another heat exchanger, E, using water or air as the coolant, F. It is then fed back to the primary heat exchanger to repeat the cycle. Waste hydrothermal fluid, C, can be reinjected into the producing field.

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

Zeolites with lower UCS are initially less active than the conventional rare earth exchanged zeolites (Figure 3-5). However, the lower UCS zeolites tend to retain a greater fraction of their activity under severe thermal and hydrothermal treatments, hence the name ultrastable Y. 

A typical NaY zeolite contains approximately 13 wt% Na20. To enhance activity and thermal and hydrothermal stability of NaY, the sodium level must be reduced. This is normally done by the ion exchanging of NaY with a medium containing rare earth cations and/ or hydrogen ions. Ammonium sulfate solutions are frequently employed as a source for hydrogen ions. 

A negative correlation between Mg content and Ca content of hydrothermally altered basalt and dacite from the Kuroko mine area exists. This correlation indicates that Ca in the rocks is removed to fluid by the exchange of Mg in seawater. Eu may behave in the manner similar to Ca during seawater-volcanic rock interaction because of the similarity of their ionic radii. 

M.J. and Holland, H.D. (1978) Chemical exchange during hydrothermal alteration of basalt by seawater. I Experimental results for major and minor components of seawater. Geochim. Cosmochim. Acta, 42, 1103-1115. 

The As (arsenic) concentration of seawater is controlled by input of rivers, sedimentation on the seafloor, weathering of the seafloor, exchange between atmosphere and seawater, volcanic gas input, and hydrothermal input. Previous studies on the geochemical cycle of As have not taken into account the hydrothermal flux of As. Therefore, hydrothermal flux of As from back-arc, island arc and midoceanic ridges to ocean is considered below. 

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