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Hydroxyl-bastnasite-(Ce), ideally CeC[O.sub.3](OH), had been regarded isostructural with Nistnasite-(Ce), CeC[O.sub.3]F, the dominant member of the bastnasite family that produces ~70% of the world's supply of rare-earth elements. Using single-crystal X-ray diffraction and Raman spectroscopy, our structural analysis on hydroxyl-bastnasite-(Ce) shows that the previous assumption is incorrect. The crystal structure of hydroxyl-bastnasite-(Ce) possesses P[bar.6] symmetry with unit-cell parameters a = 12.4112(2), c = 9.8511(3) [Angstrom], and V- 1314.2(1) [[Angstrom].sup.3], in contrast to the space group P[bar.6]2c and a [approximately equal to] 7.10, c [approximately equal to] 9.76 [Angstrom] and V [approximately equal to] 430 [[Angstrom].sup.3] for bastnasite-(Ce). Moreover, there are 6, 3, and 5 symmetrically-distinct C[O.sub.3] groups, Cecations, and (OH/F) ions, respectively, in hydroxyl-bastnasite-(Ce), but 1, 1, and 2 in bastnasite-(Ce). The two structures, nevertheless, are similarly characterized by the layers of C[O.sub.3] groups alternating with the Ce-(OH/F) layers along the c direction. The Raman spectrum of hydroxyl-bastnasite-(Ce) is dominated by three strong bands at 1080, 1087, and 1098 [cm.sup.1] in the C[O.sub.3] symmetrical stretching region, along with at least four bands in the OH stretching region. Our study further suggests that natural hydroxyl-bastnasite-(Nd) is most likely isotypic with hydroxyl-bastnasite-(Ce), rather than with bastnasite-(Ce), as previously proposed. Keywords: Bastnasite, hydroxyl-bastnasite-(Ce), single-crystal X-ray diffraction, crystal structure, Raman spectra

Ca-bearing silicate garnets are synthesized at high pressure and have tetragonal symmetry due to the ordering of Mn and Si on octahedral sites. Ordering of Ca into the dodecahedral site D2 minimizes Ca-Ca interactions at high temperature and pressures. Ca-bearing majorites such as HgSiO3 majorite exhibit mesohedral twinning by two-fold rotation.

X-ray intensities for the 'tetragonal almandine-pyrope phase' (TAPP), *[(FeMg).sup.VIII][(AlMgFe).sub.4][[.sup.IV](SiAl)[O.sub.4]].sub.3] where * refers to a capped tetrahedral site, reported by Harris et al. (1997) were recollected using the identical crystal, and the crystal structure was refined from these new data. The basic structure of the previous report is confirmed; however, some of the details regarding cation occupancies are revised. The chemical composition of this phase corresponds to a high-pressure origin; however, significant questions remain regarding the crystallization pressure of this phase.

Hydroxyl-bästnasite-(Ce), ideally CeCO3(OH), had been regarded isostructural with bästnasite-(Ce), CeCO3F, the dominant member of the bästnasite family that produces ~70% of the world's supply of rare-earth elements. Using single-crystal X-ray diffraction and Raman spectroscopy, our structural analysis on hydroxyl-bästnasite-(Ce) shows that the previous assumption is incorrect. The crystal structure of hydroxyl-bästnasite-(Ce) possesses P6̄ symmetry with unit-cell parameters a = 12.4112(2), c = 9.8511(3) Å, and V = 1314.2(1) ų, in contrast to the space group P6̄2c and a ≈ 7.10, c ≈ 9.76 Å, and V ≈ 430 ų for bästnasite-(Ce). Moreover, there are 6, 3, and 5 symmetrically-distinct CO3 groups, Ce cations, and (OH/F) ions, respectively, in hydroxyl-bästnasite-(Ce), but 1, 1, and 2 in bästnasite-(Ce). The two structures, nevertheless, are similarly characterized by the layers of CO3 groups alternating with the Ce-(OH/F) layers along the c direction. The Raman spectrum of hydroxyl-bästnasite-(Ce) is dominated by three strong bands at 1080, 1087, and 1098 cm-1 in the CO3 symmetrical stretching region, along with at least four bands in the OH stretching region. Our study further suggests that natural hydroxyl-bästnasite-(Nd) is most likely isotypic with hydroxyl-bästnasite-(Ce), rather than with bästnasite-(Ce), as previously proposed. [ABSTRACT FROM AUTHOR]