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| Holland T. J. B., Am. Mineral. 65, 129 (1980). |
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| Hollandite-type compounds possess the general chemical formula A x B 8 O 16 where A represents large mono- or divalent cations with x ≤ 2 and B represents small two- to five-valent cations. A. E. Ringwood et al. [ Acta Crystallogr. 23 1093 (1967)] transformed sanidine KAlSi 3 O 8 into the hollandite structure with Al and Si occupying octahedral sites. It was the second silicate after SiO 2 -stishovite to display sixfold coordination of Si. All of the compounds with the hollandite structure have tetragonal symmetry. |
| NaAlSi 3 O 8 -hollandite was first synthesized by L. Liu [ Earth Planet. Sci. Lett. 37 438 (1978)]. |
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| Chen, M., Sharp, T. G., Goresy, A. E., Wopenka, B., Xie, X. (1996) The Majorite-Pyrope + Magnesiowustite Assemblage: Constraints on the History of Shock Veins in Chondrites. Science, 271 (5255). 1570-1573 doi:10.1126/science.271.5255.1570 |
| . These authors have provided a description of the meteorite and its mineralogy. |
| The high-pressure polymorphs of olivine α-(Mg Fe) 2 - SiO 4 and the (Mg Fe)SiO 3 -majorite phases have been reported in many meteorites [see A. J. Brearley and R. H. Jones in Planetary Materials vol. 36 of Reviews in Mineralogy J. J. Papike Ed. (Mineralogical Society of America Washington DC 1998) pp. 1–398]. |
| (Mg Fe)SiO 3 -ilmenite has been discovered in the Tenham meteorite [ |
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| (Mg Fe)SiO 3 -perovskite has been discovered in the Tenham meteorite [ |
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| D. Stöffler A. Bischoff V. Buchwald A. E. Rubin in Meteorites and the Early Solar System J. F. Kerridge and M. S. Matthews Eds. (Univ. of Arizona Press Tucson 1988) pp. 165–202. |
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| A. El Goresy B. Wopenka M. Chen G. Kurat Meteoritics 32 A38 (1997) |
| Raman spectra were recorded with a Dilor XY spectrometer equipped with confocal optics and a nitrogen-cooled charge-coupled device (CCD) detector. A microscope was used to focus the excitation laser beam (488- and 514-nm lines of a Spectra-Physics Ar + laser) to a 2-μm spot and to collect the Raman signal in the backscattered direction. Accumulations lasting from 120 to 300 s were made. The laser power was 2 to 50 mW to avoid deterioration of the sample. |
| Seifert F. A., Mysen B. O., Virgo D., Am. Mineral. 67, 696 (1982). |
| Pieces of the M'bale L6 chondrite have been pressurized to 22 GPa and 1500 K in a multianvil press [high-pressure facility at the Bayerisches Geoinstitut (Bayreuth Germany)] in the stability field of KAlSi 3 O 8 -hollandite. Nearly pure KAlSi 3 O 8 -orthoclase of the starting material has been transformed to a new phase presumably KAlSi 3 O 8 -hollandite. The Raman spectrum of this phase (Fig. 2) is different from that of orthoclase which is characterized by strong bands near 500 and 950 cm –1 characteristic of stretching vibrations of SiO 4 tetrahedra. The Raman spectra of KAlSi 3 O 8 - and NaAlSi 3 O 8 -hollandite resemble that of stishovite [ |
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| ]. The small difference in the Raman frequencies between (Ab 80 An 12 Or 8 )- and KAlSi 3 O 8 -hollandite is accounted for by the small difference in cell parameters (see Table 3). |
| The x-ray facility used in this study includes a rotating anode generator (18 kW) a capillary collimating system and a CCD area detector. The radiation from the rotating anode with a molybdenum target is filtered by a zirconium foil so that the intensity of Kβ is 1% of that of Kα. The beam of initial size 1 mm by 0.5 mm is collimated to 0.1-mm diameter using the capillary system. A special collimator is used to reduce the size of the x-ray spot to 40 μm full width at half maximum. The diffracted x-rays were collected on a 512 by 512 pixels area detector. Data were acquired at fixed 2θ settings of 15 25 and 30 and a sample-to-detector distance of 210 mm. Collection time in different points varied from 15 min to 12 hours. Settings of the detector were calibrated with three external independent standards (W MgO Al 2 O 3 ) at each position of the detector. The sample disk was mounted on a 0.4-mm hole in a larger steel disk that was loaded onto the goniometer stage. We rotated the sample plate 30° from the initial position normal to the x-ray beam with a step of 1° in the ω axis during data collection. The position of the collimated x-ray beam penetrating through the hollandite grain was continuously monitored on a screen using a CCD camera. |
| Zhang J., Ko J., Hazen R. M., Prewitt C. T., Am. Mineral. 78, 493 (1993). |
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| M.C. is supported by NSF of China grant 49825132 and Deutsche Forschungsgemeinschaft grant Go 315/15-1. This work was supported by the Programme National de Planétologie (CNRS-INSU). G. Montagnac helped in the Raman measurements and B. Reynard and R. Hemley substantially improved our understanding of the Raman data. |
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