Crystal structure and the 57Fe Mössbauer spectrum of synthetic (Ca1.53Na0.47)(Mg0.52Fe3+0.49)Si1.99O7-melilite have been investigated to confirm the location of Fe3+ at the T1 site, and to evaluate the effect of an incommensurate structure on 57Fe Mössbauer hyperfine parameters. A conventional Rietveld refinement with occupancies of Ca(W), Na(W), Mg(T1), Fe3+(T1) and Si(T2) fixed to 0.765, 0.235, 0.515, 0.485 and 2.00, respectively, converged well with an Rwp (R-weighted pattern) of 13.14% and a goodness-of-fit of 1.337, indicating exclusive Fe3+ distribution at the T1 site. However, the Mössbauer spectrum consists of two doublets with an isomer shift of 0.19 mm/s and quadrupole splitting of 0.75 and 1.04 mm/s, which can be attributed to Fe3+ at the T1 site. There are two sorts of T1 site with distinguishable distortion in the synthetic Na-Fe3+-melilite, which might be attributed to the existence of an incommensurate phase at room temperature.
Jadeitite exhibiting jadeite with quartz inclusions has been newly found in the Nishisonogi metamorphic rocks, Kyushu, Japan. The jadeitites occur in boulders from a riverbed, together with boulders of albitites, clinozoisite-muscovite rocks, and serpentinites. The distribution of these boulders is confined to an area that is downstream from an outcrop of a serpentinite melange, suggesting that they were originally tectonic blocks in this serpentinite melange. The jadeitites consist mainly of jadeite with small amounts of muscovite, paragonite, phlogopite, albite, analcime, clinozoisite, and titanite. The jadeite consists of a core with abundant inclusions of quartz and omphacite, and a rim that is free from quartz inclusions. The quartz inclusions are in direct contact with the host jadeite, which has an almost pure NaAlSi2O6 composition (Jd100-Jd95). The volume fraction of the quartz inclusions (VQtz/(VJd + VQtz) = 0.20-0.28) in the jadeite core is close to the value (VQtz/(VJd + VQtz) = 0.27) of quartz produced by the reaction albite = jadeite + quartz. These findings suggest that the jadeite core was produced by an isochemical breakdown of albite at high-P/T conditions. In addition, the jadeite is locally replaced by albite and/or analcime at the rim and along microfractures. These microtextures provide information to deduce a retrograde P-T path during the exhumation of the jadeitite.
The a and c unit cell dimensions of melanophlogite (MEP) have been determined in the temperature range −50 to 700 °C, showing a different expansion behavior for the low temperature α-phase. The c-axis length, 2c, which is smaller than a-axis length in the α-phase, shows a steep rise reaching the value of a at the tetragonal (α)-cubic (β) transition temperature at about 65 °C, and then remains nearly constant until about 500 °C, after which contraction occurs. The crystal structures of α-MEP (space group = P42/nbc) and β-MEP (space group = Pm3n) were refined using a least-squares refinement of a harmonic structure factor expression, using single crystal X-ray diffraction data measured at seven temperatures from −50 to 200 °C (four points for the α-phase, and three points for the β-phase). The average Si-O distance decreased from 1.593 Å at −50 °C down to 1.573 Å at 63 °C (the estimated transition point). It then remained nearly constant in the β-phase. The Si-O bond distance corrected using a simple rigid body motion model remained nearly constant at 1.611 Å in the temperature range −50 to 200 °C, indicating that the negative temperature dependence is due to strong distortions of the probability density functions of the O atoms. The atomic mean-square displacement, <u2>, of the O atoms increased steeply with increasing temperature up to the α-β transition point. The low-high (α-β) transformation in MEP is driven by a mechanism involving atom disorder beginning in the low-temperature phase.
The presence of abundant phyllosilicates in many carbonaceous chondrites indicates a prevailing activity of low-temperature aqueous alteration in primitive asteroids. However, among the hydrous carbonaceous chondrites known, more than 20 samples show evidence of having been heated at elevated temperatures with corresponding phyllosilicate dehydration. The mineralogical features of dehydration suggest that the heating occurred in situ in meteorites, which demonstrates that there are some hydrated asteroids that have been heated at a certain period after aqueous alteration. Recent studies have uncovered details of heating and dehydration processes in hydrous carbonaceous chondrites: step-by-step changes in mineralogy, trace element chemistry, carbonaceous materials, and reflectance spectra have been clarified. Based on data from synchrotron X-ray diffraction analysis of the matrix, heated hydrous carbonaceous chondrites have been classified as Stages I-IV, with the temperature of heating increasing from I to IV. In spite of recent progress, heat sources are poorly defined, mainly due to a lack of chronological information on the timing of the heating, and therefore more data are needed to fully clarify the thermal metamorphism of hydrous carbonaceous chondrites.