SUMMARY, Phosphate minerals are widespread minor constituents of the pallasites. Both anhydrous primary and hydrous decomposition/alteration phosphates occur. The primary minerals, in decreasing order of abundance, are whitlockite, stanfieldite, and farringtonite, as well as a possibly new phosphate. The decomposition phosphates are compositionally heterogeneous and thus complex. Some may be new minerals. The phosphate minerals serve as the major repository for the alkali elements in pallasites, with individual crystals containing up to several percent of Na+K. Equilibrium established between coexisting phosphide and phosphate defines a relatively narrow range offo. values.PAL LAS I T I C meteorites are essentially composed of Fe-Ni metal and olivine, with minor troilite; they have been considered remarkably simple mineralogically. Olivine is the only silicate that has been reported, although pyroxene has recently been observed as a minor phase (Buseck, I977). DuFresne and Roy (I961) described farringtonite (essentially Mg3(PO4)~) from the Springwater pallasite and Fuchs (1969) reported stanfieldite (Ca~(Mg,Fe)5(PO4)6) and whitlockite (ideally Caz(PO4)2) from a few pallasites.As part of a comprehensive study of the mineralogy and petrology of the pallasites, we have observed several phosphate minerals. They include the three phosphates mentioned above, as well as a few additional ones. Several of the latter, including ones containing small amounts of Ni or Cr, probably result from alteration of the primary pallasite minerals.We have observed phosphates in 31 of 4I pallasites studied; it appears that they are essential, albeit minor, constituents of pallasitic meteorites. In addition to the phosphates, several other minor phases occur (Buseck, I968, I969).The phosphate minerals are significant phases in pallasites for several reasons. Not only are they widespread, but they play an important role in that they ser~ve as repositories for elements such as Na, K, U, and Th, which are not accommodated within the major minerals. Additionally, coexisting phosphates and phosphide can be used to define the fairly narrow range of fo2 conditions under which the pallasites formed. The existence of a suite of anhydrous phosphates, rare in terrestrial mafic igneous rocks, is one of several features giving uniqueness to the pallasites.
OccurreflcffIt is likely that phosphate minerals occur in all of the pallasites, for in many cases we have only been able to study single small (e. 1 cm 2) polished sections from a given meteorite; such a small sample cannot be representative of these coarse-grained meteorites. Consequently we believe it probable that the pallasites that we were either unable to examine, or in which we observed no phosphates, actually do contain such phases.