New Alkali Phosphates and Their Associations in the IIIAB Iron Meteorites

Details New Alkali Phosphates and Their Associations in the IIIAB Iron Meteorites New Alkali Phosphates and Their Associations in the IIIAB Iron Meteorites

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.415o&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 415

Computer Science

Iiiab Iron Meteorites, Minerals, Phosphates

Scientific paper

The IIIAB iron meteorites commonly contain inclusions of phosphate + oxide minerals within troilite nodules (occasionally directly within metal). The most frequently encountered associations are chromite with (1) graftonite, (Fe,Mn)(sub)3(PO(sub)4)(sub)2 (with Mn usually <4 mol%, but occasionally up to 50 mol%); (2) sarcopside (a polymorph of graftonite) with graftonite; or (3) sarcopside with beusite, (Fe,Mn)(sub)3(PO(sub)4)(sub)2 (Mn >= 50 mol%]. In addition, any of these three associations may contain one or two of three alkali-bearing phosphates, two of which appear to be new minerals: johnsomervilleite Na(sub)2Ca(Fe,Mn)(sub)7(PO(sub)4)(sub)6 (Fe >> Mn); new phase #1 (Na,K)(sub)2(Fe,Mn)(sub)8(PO(sub)4)(sub)6 (Fe >> Mn, Na > K); new phase #2 (K,Na)(sub)2(Fe,Mn)(sub)8(PO(sub)4)(sub)6 (Fe >> Mn, K > Na). Crystallographic work has been undertaken on these latter two phases in preparation for proposing them as new minerals. For the time being they are designated the 2:8:6 phosphates. Johnsomervilleite is a 2:1:7:6 phosphate; graftonite/sarcopside and beusite are 9:6 phosphates. Johnsomervilleite is the Fe end member of the new 2:1:7:6 alkali-Mg phosphate mineral, Na(sub)2 Ca(Mg,Fe,Mn)(sub)7(PO(sub)4)(sub)6, found in a silicate inclusion in the Carlton IIIC iron meteorite [1]. Five assemblages, all of which contain chromite, have been investigated thus far: Chupaderos (IIIB) with graftonite + johnsomervilleite; Grant (IIIB) with sarcopside + beusite + johnsomervilleite + trace of silica; Bella Roca (IIIB) with sarcopside + beusite + 2:8:6 [Na/(Na + K) = 0.97]; El Sampal (IIIA) with sarcopside + beusite + johnsomervilleite + 2:8:6 [Na/(Na + K) = 0.98]; Sandtown (IIIA) with graftonite (with up to 3 wt% CaO) + johnsomervilleite + K-rich 2:8:6 ([Na/(Na + K) = 0.40]. The 2:1:7:6 phosphates can be considered to have three end members: fillowite Na(sub)2CaMn(sub)7(PO(sub)4)(sub)6; johnsomervilleite Na(sub)2CaFe(sub)7 (PO(sub)4)(sub)6; new phase Na(sub)2CaMg(sub)7(PO(sub)4)(sub)6 [1]. The similarity of the X-ray diffraction patterns of all three suggests that their crystal structures are the same as that of fillowite [2]. No X-ray data have yet been obtained for the 2:8:6 phosphates. The lack of Ca, however, may indicate that their structures either differ from that of the 2:1:7:6 phosphates or that Fe can substitute in the Ca site in the fillowite structure. Because both the 2:8:6 and 2:1:7:6 phases coexist in at least two phosphate assemblages (Sandtown and El Sampal), we believe that the 2:8:6 phase has a different structure than that of the 2:1:7:6 phosphates. The Mg and Mn end members of the 2:8:6 trio have not been recognized, but based on the crystallographic similarity of Mg, Mn, and Fe, they are probable minerals given the right chemical environment. Tentatively, we consider the Na- and K- rich 2:8:6 phases may be an isostructural solid solution above some temperature, but forming separate K-rich and Na-rich phases at lower temperatures if the total K content is high enough. In the IIIAB irons we believe that these phosphates form from trace components (including oxygen) occluded from metal during its solidification and reaction with Fe and P components. References: [1] McCoy and Keil, personal communication. [2] Araki T. and Moore P. B. (1981) Am. Mineral., 66, 827-842.

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