Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
http://pubs.acs.org/journal/aesccq
Formation of Hydrous, Pyroxene-Related Phases from LiAlSiO4 Glass in High-Pressure Hydrothermal Environments Jonas Ångström,† Istvan Zoltan Jenei,† Kristina Spektor,‡ and Ulrich Häussermann*,† †
Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden European Synchrotron Radiation Facility (ESRF), 38000 Grenoble, France
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ABSTRACT: Hydrous Al-bearing pyroxene-related phases were synthesized by subjecting LiAlSiO4 glass to hydrothermal environments at pressures of 5−10 GPa and temperatures of 400−600 °C. LiAlSiO3(OH)2 formed at 5 GPa, whereas at 10 GPa, product mixtures of LiAlSiO3(OH)2 and Li3Al4(Si2O7)(SiO3)2(OH)5 were obtained. The monoclinic structure of LiAlSiO3(OH)2 has been previously characterized from single-crystal X-ray diffraction data (Spektor, K.; Fischer, A.; Häussermann, U. Crystallization of LiAlSiO4 Glass in Hydrothermal Environments at Gigapascal Pressures−Dense Hydrous Aluminosilicates. Inorg. Chem. 2016, 55 (16), 8048−8058, 10.1021/acs.inorgchem.6b01181). It resembles that of α-spodumene (LiAlSi2O6) and constitutes alternating layers of chains of corner-condensed SiO4 tetrahedra and chains of edgesharing AlO6 octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO4 tetrahedron chain layers. The structure solution of Li3Al4(Si2O7)(SiO3)2(OH)5, as detailed here, was achieved by rotational electron diffraction analysis, and the model was refined against synchrotron powder X-ray diffraction data (space group C2/c, a = 4.921 Å, b = 25.849 Å, c = 9.170 Å, and β = 99.42°). The crystal structure of Li3Al4(Si2O7)(SiO3)2(OH)5 features chains and pairs of corner-condensed SiO4 tetrahedra, with the Si atoms equally distributed among the two structural units, and thus Li3Al4(Si2O7)(SiO3)2(OH)5 is a rare example of a mixed inosorosilicate. LiAlSiO3(OH)2 and Li3Al4(Si2O7)(SiO3)2(OH)5 are structurally closely related to recently discovered hydrous magnesium aluminosilicate phases (i.e., HAPY and HySo), which form at conditions similar to the hydrous lithium aluminosilicates. The conjecture is made that hydrothermal environments following chlorite but also lawsonite breakdown generally afford conditions for the formation of hydrous, pyroxene-related, aluminosilicate phases, with compositions of M21−mM1TO3+n(OH)2−o (0 < m, n, and o < 1). These phases could be transients in breakdown reactions but also stable at cold slab conditions and, thus, may play an important role to water storage and transport to the transition zone. KEYWORDS: lithium aluminum silicates, hydrous pyroxene-like structures, rotational electron diffraction, multi-anvil synthesis, water transport in the mantle
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INTRODUCTION Water transport into the transition zone via subducting slabs involves a complex sequence of initial hydration and subsequent dehydration (breakdown) reactions of mostly layered−structured minerals that carry water as hydrates and hydroxides. A pertinent question is the role of the so-called dense hydrous magnesium silicates (DHMS), which supersede layered−structured minerals (e.g., clinochlore/chlorite, Mg5Al2Si3O10(OH)8; serpentine, Mg3Si2O5(OH)4; and lawsonite, CaAl2Si2O10H4) as water carriers in the upper mantle.1,2 DHMS include over half a dozen mineral phases, e.g., “10 Å” phase and phases “A” and “B”; some of them may carry water even down to lower mantle regions.3,4 Additionally, Al substitution of Si in DHMS phases can noticeably increase their stability in the lower mantle, creating the possibility for hosting large water reservoirs within it.5 Metamorphic relations of OH-bearing minerals over the range of upper to lower © XXXX American Chemical Society
mantle p and T conditions as well as associated mechanisms and kinetics of their breakdown reactions are currently debated.6 The mineralogy of the mantle is complex and still full of surprises, such as, for example, recently manifested by the discovery of the water-rich Mg silicate phase (Mg,Si)O2(OH)2 (phase H).7,8 This phase appears to be stable up to 50 GPa and, thus, provides potentially unforeseen pathways for even deeper recycling of water into the lower mantle. Likewise, interesting are recent reports of new phases in the MgO− Al2O3−SiO2−H2O (MASH) system,9−11 which were prepared at conditions emulating clinochlore/chlorite breakdown in the Received: Revised: Accepted: Published: A
July 10, 2018 October 23, 2018 December 4, 2018 December 4, 2018 DOI: 10.1021/acsearthspacechem.8b00091 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry
SEM Investigations. SEM imaging and energy-dispersive X-ray spectroscopy (EDS) was performed using a JEOL JSM 7000F microscope equipped with a Schottky-type field emission gun. For imaging, powder samples were dispersed over a sticky carbon tape mounted on an aluminum stub and partially coated with a 10−15 nm gold layer to decrease charging. An accelerating voltage of 2−3 kV and a probe current of
