Phosphate in Angra dos Reis: Structure and composition of the Ca3(PO4)2 minerals
| Autor: | Eric Dowty |
|---|---|
| Rok vydání: | 1977 |
| Předmět: |
Mineralogy
chemistry.chemical_element Yttrium Calcium engineering.material Phosphate chemistry.chemical_compound Crystallography Geophysics chemistry Space and Planetary Science Geochemistry and Petrology Chondrite Earth and Planetary Sciences (miscellaneous) Anhydrous Whitlockite engineering Composition (visual arts) Achondrite Geology |
| Zdroj: | Earth and Planetary Science Letters. 35:347-351 |
| ISSN: | 0012-821X |
| Popis: | Extraterrestrial calcium phosphates (“whitlockites”) have the anhydrous β-Ca3(PO4)2 structure, which is different from that of hydrous terrestrial whitlockite. This has been confirmed by X-ray refinement of the structure of a phosphate from the achondrite Angra dos Reis. In the β-Ca3(PO4)2 structure, there is one crystallographic site, Ca(IIA), which is half-occupied by calcium, and which seems to have an energetically unfavorable configuration; natural phosphates with this configuration (including Angra dos Reis) have composition Ca19(Mg,Fe)2(PO4)14. Stability of the structure is probably increased by substitution of Na for Ca in Ca(IIA) giving composition Ca18 (Mg,Fe)2Na2(PO4)14, which occurs in chondrites; by vacancy of Ca(IIA), with rare earths and yttrium substituting for calcium in other sites for charge balance, giving composition Ca16(Y,RE)2(Mg,Fe)2(PO4)14, found in lunar rocks; or by replacing Ca with hydrogen, giving composition Ca18(Mg,Fe)2H2(PO4)14, which is terrestrial whitlockite. Lack of the favorable substitutions of Na, (Y, RE) or H in Angra dos Reis phosphate implies that these elements were relatively scarce in its environment of formation. |
| Databáze: | OpenAIRE |
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