Thermodynamics of mixing for solids and liquids in the anhydrous apatite system Ca$\sb3$(PO$\sb4)\sb2$ - CaF$\sb2$ - Ca(OH)$\sb2$ - CaCl$\sb2$ are quantified using mathematical modeling of new and existing experimental data. These data apply to apatite experiments or to understanding natural apatites.
Fluorapatites (FAp) are hexagonal, although hydroxyapatite and chlorapatite (ClAp) are monoclinic. Although the monoclinic and hexagonal endmembers are believed to be immiscible, no evidence of immiscibility exists. Miscibility is achieved by movement of the anions away from the sites occupied in the end member and by expansion of the apatite lattice. Anion-anion interaction and lattice deformation may lead to possible nonideal mixing of the endmembers. Thermodynamic treatment of experimental data gathered above 475$\sp\circ$C shows that apatite solid solutions may be considered ideal. Standard state data for chlorapatite are derived.
Thermodynamic modeling of the molten mixtures uses a stoichiometric approach with Ca$\sb{1.5}$PO$\sb4$ and Ca$\sb{0.5}$X (X = F, OH, Cl) liquid phase components. New data are presented for joins Ca(OH)$\sb2$ - CaCl$\sb2$ and CaF$\sb2$ - Ca(OH)$\sb2$. Molten mixtures on the join CaF$\sb2$ - CaCl$\sb2$ are ideal. Ca(OH)$\sb2$ - CaCl$\sb2$ and CaF$\sb2$ - Ca(OH)$\sb2$ molten mixtures show positive enthalpies of mixing typical of common cation-mixed anion systems. The phosphate-salt joins show small positive entropies of mixing due to formation of polymeric species in the phosphate melt not explicitly considered in the model. Heats of fusion for solids and standard state data for the liquid phase components, CaClF, CaOHCl and Ca$\sb2$PO$\sb4$Cl are derived.
The apatite thermodynamic data are applied to existing data for the Bishop Tuff (BT) and new data for the Fish Canyon Tuff (FCT). Results show that both magmas were saturated with a fluid phase prior to eruption, in agreement with earlier studies. Apatite inclusions in FCT phenocrysts preserve the pressure- temperature path of the magma, requiring a higher pressure for plagioclase crystallization than for hornblendes. Similar treatment of the BT shows that the pressure paradox observed by earlier workers is also found in the apatite chemistry. Applications are limited currently by analytical problems, but the ratio of X$\sb{\rm FAp}$/X$\sb{\rm ClAp}$ is probably accurate.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/16560 |
| Date | January 1992 |
| Creators | Tacker, Robert Christopher |
| Contributors | Stormer, J. C., Jr. |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | 185 p., application/pdf |
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