Other
Scientific paper
Jun 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977e%26psl..35..347d&link_type=abstract
Earth and Planetary Science Letters, vol. 35, no. 2, June 1977, p. 347-351.
Other
13
Extraterrestrial Matter, Meteoritic Composition, Minerals, Phosphates, Calcium Phosphates, Chondrites, Crystallography, Rare Earth Compounds
Scientific paper
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 swere relatively scarce in its environment of formation.
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