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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

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.08.019 Byline: Susanne Douglas, William Abbey, Randall Mielke, Pamela Conrad, Isik Kanik Keywords: Astrobiology; Mars; surface; Mineralogy; Geological processes Abstract: For in situ astrobiological studies of Mars or other planets, we must employ strategies that will enable us to verify whether our approach and prototype instruments are actually capable of distinguishing life from non-life. This must be done against a background of rigorously conducted scientific characterization of the environment or sample types being considered for measurement by the instruments under development. In this study we show how a combination of mineralogical and textural features can be considered a biosignature in an early Mars analogue environment, Death Valley, California. We propose that it is a combination of features in context of the geologic matrix which allows determination of biogenicity to be made. Polymineralic microbialites (organosedimentary formations constructed by microorganisms) from a spring pool at Badwater, within Death Valley National Park, are composed of alternating biogenic and abiogenic minerals in a distinct triplet sequence related to wet and dry seasons. A microbial community, occurring as a black biofilm, produced paired layers of two different mineral types: manganese oxyhydroxides and calcite. These biogenic layers are separated from the next pair by a gypsum layer and appear to be laid down in the wet season, with the gypsum (a mineral positively identified on Mars) precipitating in the dry part of the year, abiogenically (i.e., not dependent on microbial metabolic activity for its deposition). In addition, textural features (smaller grain size and less geometric morphology) unique to the biogenic vs the abiogenic layers, were consistently observed so that texture served as a biosignature in this environment. Author Affiliation: California Institute of Technology, Jet Propulsion Laboratory, MS 183-301, 4800 Oak Grove Dr., Pasadena, CA 91109-8099, USA Article History: Received 9 October 2006; Revised 10 August 2007

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.

Brillouin spectroscopy was used at pressure to obtain elastic stiffness constants of pyrope, grossular and andradite, natural garnets. Acoustic velocities were measured at pressures to 10 GPa in a diamond-anvil cell. Cauchy relations altered with pressure for grossular and andradite, indicating that they are constant and not suitable for extrapolation to high pressures.

Silicate pyroxenes are important parts of the Earth's crust, upper mantel and meteorites. High-pressure structures studies on orthopyroxene, low-pressure clinopyroxene and high-pressure clinopyroxene have been undertaken. A new systematic high-pressure structure study on protopyroxene has been performed, using single-crystal X-ray diffraction and Raman spectroscopy. The Pbcn protopyroxene structure transforms to a new pyroxene structure having P2(sub1)cn symmetry at high pressure.