Structural Comparison of Lunar , Terrestrial , and Synthetic

Raman spectra are sensitive to the structural and chemical characteristics of the phosphate mineral whitlockite. The number and specific frequency of the Raman bands in the 970 A m ' spectral region caused by the vl symmetric stretching mode within the P043tetrahedra differ for REE-rich whitlockite (single, asymmetric band), REE-poor Caor Na-whitlockite (doublet), and terrestrial Hantaining whitlockite (doublet). The presence of HP02groups in the latter produces a third v band at 923 Acm-l. Based on the Raman spectra of lunar, meteoritic, synthetic, and terrestrial whitlockite, we conclude that &e incorporation of REE in REE-rich whitlockite (for which there is currently no X-ray structural refinement) causes a significant change to the structure compared to both meteoritic and terrestrial whitlockite. In addition, we can distinguish between high and low REE concentrations in whitlockite grains. All of this information can be obtained nondestructively, in-situ in grains as small as 3 pn (in loose rock fragments or in prepared sections) with a laser Raman microprobe. Introduction and Crvstal Chemistry. Whitlockite (a.k.a. merrillite) is an important accessory mineral in lunar rocks and meteorites and has been the subject of numerous recent studies of petrogenesis [e.g., 1, 2, 3 meteorites; and 4, 5 lunar rocks]. It is one of the two most common phosphates in extraterrestrial rocks, the other being apatite. Both of these minerals have strong and distinctly different Raman spectra. In the whitlockite structure, there are three symmetrically distinct phosphate tetrahedra: per 56 oxygens, there are two P(A), six P(B1) and six P(B2) groups (using the nomenclature of [6]). Extraterrestrial whitlockites differ from their terrestrial counterpart by enrichments in Ca2+, Na' or FEE3+, and a total lack of H+ (Table 1). Structural studies by X-ray diffraction of terrestrial whitlockite [6] and meteoritic whitlockite [7, 81 showed that the essential structural difference is an inversion of the P(A) tetrahedra, which creates an additional octahedral site in the H-freeJ?-Ca3(POJ2 structure. It is this Ca@A) site (whose partial occupancy by ca2+, Na', or a vacancy coupled with REE3+ substitution on the other Ca sites) that allows for a range of compositional variability in the lunar and meteoritic whitlockites (4, 8, 9; Table 1). Based on this structural difference, it was recommended [6, 81 that H-free whitlockites be considered a different mineral and called merrillite; in this abstract, we refer to the samples for simplicity as whitlockite (terrestrial), REE-whitlockite (lunar & synthetic), and Caor Na-whitlockite (meteoritic & synthetic). Whitlockite Spectra. We have obtained Raman spectra (Fig. 1) of lunar REE-whitlockite, synthetic REE-rich and REE-poor whitlockite, and terrestrial whitlockite (Table 1). All of these spectra have prominent Raman peaks due to vibrations within the phosphate tetrahedra. The strongest bands in the 950-975 Am-' region are associated with the vl symmetric stretching vibration of the phosphate group. Other features shared by all of the spectra include weak bands in the 1080-1 100 Acm-I region (v3 asymmetric stretching vibration), in the 400-500 A m ' region (v2 bending mode), and in the 550-660 ~ c m I region (v4 bending mode). Weak bands at less than 400 Am-' are lattice modes. In the terrestrial H-bearing whitlockite and the REE-poor synthetic Ca-whitlockite (Figs. la,b), the strong vl band is a wellresolved doublet, as it is also in Na-whitlockite of the Sixiangkou chondrite, which is the only other published Rarnan spectrum of whitlockite to date [lo]. However, in lunar REE-whitlockite and REE-rich synthetic whitlockite, this band is an asymmetric single peak and a very poorly resolved doublet, respectively (Figs.lc, d). The terrestrial whitlockite has an additional weak peak at 923 Acm-I that is not observed in the spectra of the H-free whitlockites; this we attribute to the v mode of HP02groups involving P(A) tetrahedra. In general, Raman bands of hydroxyl ions occur as sharp, well defmed bands at or above 3200 Acm-I. However, the Raman-active 0-H stretch of the weakly acidic hydroxyl group in HP02is predicted to be broad and very weak, located in the 2100-2700 ~ c m I region of the vibrational spectrum. Unfortunately, we could not locate this band in the terrestrial whitlockite sample (Fig. la), possibly because of the weak background fluorescence observed in this spectral region. In addition to the Rarnan bands discussed above, we observe an increased background in the region from -1000-1500 ~ c m I in the lunar REE-whitlockite spectrum (only m a l l y seen in Fig. lc). This is attributed primarily to the laser-induced fluorescence of E$+. The Er ion has transitions low enough in energy to be excited by the 514 nm line of the Ar-ion laser. This increased background is not observed in the spectra of the synthetic whitlockites because they do not contain Er (Table 1). Discussion. Using Raman spectroscopy, we can distinguish between whitlockite that has essential (i.e., structurally necessary) hydrogen from totally anhydrous whitlockites, which is consistent with the X-ray structural analysis of those samples. In the case of the anhydrous whitlockite, the Raman spectrum also reflects a significant structural difference between REE-rich and REE-poor whitlockites, as follows. The well-resolved doublet in the terrestrial whitlockite, the meteoritic Nawhitlockite [lo], and the REE-poor synthetic whitlockite reflects at least two types of symmetrically distinct phosphate groups Table 1. Whitlockite formulae. endmemberlsample (occurrence) formula endmem bers whitlockite (terrestrial) Ca1s(MgJe)2HzP140% Ca-whitlockite (meteoritic, synthetic) Calg(Mg,Fe)2P140~ Na-whitlockite (meteoritic) Ca1dMg,Fe)zNazP140% REE-whitlockite (lunar, synthetic) C ~ I ~ R E E ~ ( M ~ , F ~ ) Z . P ~ ~ O % samples 14161,7373 (lunar)+ Ca16.sREE1.4(MgJe)2.1Nao.ssP1asSio.oQss W3-S5A3 (REE-rich, syntheti~)~ Ca16.6REE1.dMg,Fe)1.fiao.1Sla4Sio.sP% W3-S5A4a (REE-poor, synthetic)' Cale.~REEo,offMgJe)2.ONaoao1~P~3.sSio.~60~ ' In the lunar sample, "REE" includes Y. Synthetic samples include only the REE La, N 4 Sm, and Yb. Formulae of samples were calculated on the basis of 56 oxygens. Whitlockite h m the Tip Top pegmatite is assumed to have the terrestrial whitlockite formula