This chapter is concerned with in situ studies of the structural behavior of amphiboles under non-ambient conditions. Relatively few such studies have been made compared with those on samples studied under ambient conditions (room-P and T), which form the staple of many spectroscopic and diffraction studies of short-range and long-range order. Collecting data under non-ambient conditions often imposes signifi cant additional complexities in the data-collection and data-handling procedures than are necessary in ambient studies. For example, collecting high-pressure X-ray data for a sample in a diamond-anvil cell (DAC) usually means a much-reduced dataset due to limited access to reciprocal space (about 40%) imposed by the pressure cell. In order to get good coverage of reciprocal space, it is sometimes necessary to run two crystals of the same sample cut and mounted in the DAC in different orientations. The presence of pressure or temperature assemblies (DACs, multianvil devices, cryostats, furnaces) also requires the application of high-quality background corrections. Suffi ce it to say that many non-ambient in situ experiments are not trivial to perform and, in some cases, they require access to national facilities (e.g., synchrotron and neutron sources). For these reasons, non-ambient in situ studies are much less common than “quench and-look” approaches (particularly spectroscopy), which tend to focus upon compositional systematics. However, the importance of in situ studies of non-ambient behavior cannot be overstated. Many materials undergo non-quenchable displacive phase transitions at high pressure and high temperature, and these transitions can produce microtextures and be associated with anomalous thermodynamic behavior. The determination of compressibilities and expansivities provides fundamental thermodynamic data and allows the behavioral trends to be identifi ed between structurally and compositionally related phases. It is well-known that monoclinic amphiboles with signifi cant Mg occupancy of M(4) undergo a reversible displacive phase transition involving P21/m and C2/m polymorphs. For the studies reported here, which are either variable-T/room-P or variable-P/room-T, the transitions are P-induced C2/m → P21/m or T-induced P21/m → C2/m. However, recent studies have reported evidence for the existence of a high-P transition from the P21/m to a new C2/m structure which has highly-kinked (rotated) double-chains of tetrahedra, unlike the low-P/high- T C2/m structure which has almost straight chains. It will ensure clarity later if we now follow the literature and distinguish the two different C-centered amphiboles as HT-C2/m and HPC2/ m. A corresponding transition and HP-C2/c polymorph has been identifi ed in clinopyroxene containing signifi cant Mg at M(2), the pyroxene counterpart of the amphibole M(4) site. We discuss this transition in detail toward the end of the chapter. This chapter is divided into three parts. The fi rst section deals with T- and P-induced structural phase transitions in amphiboles, and focuses primarily on the P21/m ↔ HT-C2/ m transition in natural cummingtonite and synthetic ANa B(NaxLi1−xMg)2 CMg5 TSi8 O22 W(OH,F)2 amphiboles, comparing their characteristics with those of the analogous P21/c ↔ HT-C2/c transition in pyroxenes. The second section of the chapter is concerned with in situ studies of temperature-induced non-convergent cation disorder in cummingtonite and BMganalogues of richterite, with some surprising results. Finally, we fi nish with a brief discussion of compressibilities and thermal expansivities.

Non-ambient in situ studies of amphiboles / M.D. Welch, F. Cámara, G. Della Ventura, G. Iezzi - In: Amphiboles: crystal chemistry, occurrence, and health issues / [a cura di] F.C. Hawthorne, R. Oberti, G. Della Ventura, A. Mottana. - Prima edizione. - [s.l] : MINERALOGICAL SOC AMER, 3635 CONCORDE PKWY STE 500, CHANTILLY, VA 20151-1125 USA, 2007 Oct 08. - ISBN 0939950790. - pp. 223-260

Non-ambient in situ studies of amphiboles

F. Cámara
Secondo
;
2007

Abstract

This chapter is concerned with in situ studies of the structural behavior of amphiboles under non-ambient conditions. Relatively few such studies have been made compared with those on samples studied under ambient conditions (room-P and T), which form the staple of many spectroscopic and diffraction studies of short-range and long-range order. Collecting data under non-ambient conditions often imposes signifi cant additional complexities in the data-collection and data-handling procedures than are necessary in ambient studies. For example, collecting high-pressure X-ray data for a sample in a diamond-anvil cell (DAC) usually means a much-reduced dataset due to limited access to reciprocal space (about 40%) imposed by the pressure cell. In order to get good coverage of reciprocal space, it is sometimes necessary to run two crystals of the same sample cut and mounted in the DAC in different orientations. The presence of pressure or temperature assemblies (DACs, multianvil devices, cryostats, furnaces) also requires the application of high-quality background corrections. Suffi ce it to say that many non-ambient in situ experiments are not trivial to perform and, in some cases, they require access to national facilities (e.g., synchrotron and neutron sources). For these reasons, non-ambient in situ studies are much less common than “quench and-look” approaches (particularly spectroscopy), which tend to focus upon compositional systematics. However, the importance of in situ studies of non-ambient behavior cannot be overstated. Many materials undergo non-quenchable displacive phase transitions at high pressure and high temperature, and these transitions can produce microtextures and be associated with anomalous thermodynamic behavior. The determination of compressibilities and expansivities provides fundamental thermodynamic data and allows the behavioral trends to be identifi ed between structurally and compositionally related phases. It is well-known that monoclinic amphiboles with signifi cant Mg occupancy of M(4) undergo a reversible displacive phase transition involving P21/m and C2/m polymorphs. For the studies reported here, which are either variable-T/room-P or variable-P/room-T, the transitions are P-induced C2/m → P21/m or T-induced P21/m → C2/m. However, recent studies have reported evidence for the existence of a high-P transition from the P21/m to a new C2/m structure which has highly-kinked (rotated) double-chains of tetrahedra, unlike the low-P/high- T C2/m structure which has almost straight chains. It will ensure clarity later if we now follow the literature and distinguish the two different C-centered amphiboles as HT-C2/m and HPC2/ m. A corresponding transition and HP-C2/c polymorph has been identifi ed in clinopyroxene containing signifi cant Mg at M(2), the pyroxene counterpart of the amphibole M(4) site. We discuss this transition in detail toward the end of the chapter. This chapter is divided into three parts. The fi rst section deals with T- and P-induced structural phase transitions in amphiboles, and focuses primarily on the P21/m ↔ HT-C2/ m transition in natural cummingtonite and synthetic ANa B(NaxLi1−xMg)2 CMg5 TSi8 O22 W(OH,F)2 amphiboles, comparing their characteristics with those of the analogous P21/c ↔ HT-C2/c transition in pyroxenes. The second section of the chapter is concerned with in situ studies of temperature-induced non-convergent cation disorder in cummingtonite and BMganalogues of richterite, with some surprising results. Finally, we fi nish with a brief discussion of compressibilities and thermal expansivities.
displacive phase-transitions; temperature crystal-chemistry; grunerite solid-solution; X-ray-diffraction; cummingtonite-grunerite; structure refinement; powder diffraction; order-disorder; pressure; behavior
Settore GEO/06 - Mineralogia
8-ott-2007
Book Part (author)
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/478536
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