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Micro- and nano-size hydrogrossular-like clusters in pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps

Geiger, Charles A. and Rossman, George R. (2020) Micro- and nano-size hydrogrossular-like clusters in pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps. Contributions to Mineralogy and Petrology, 175 (6). Art. No. 57. ISSN 0010-7999. doi:10.1007/s00410-020-01693-1.

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The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultra-high-pressure lithologies contain coesite and nearly end-member composition pyrope, Mg₃Al₂Si₃O₁₂, making this locality petrologically and mineralogically unique. Structural OH⁻, loosely termed “water”, in pyrope crystals of different composition has been investigated numerous times, using different experimental techniques, by various researchers. However, it is not clear where the minor OH⁻ is located in them. IR single-crystal spectra of two pyropes of composition {Mg_(2.79),Fe²⁺_(0.15),Ca_(0.04)}_(Σ2.98)[Al]_(2.02)(Si)_(2.99)O₁₂ and {Mg_(2.90),Fe²⁺_(0.04),Ca_(0.02)}_(Σ2.96)[Al]_(2.03)(Si)₃O₁₂ were recorded at room temperature (RT) and 80 K. The spectra show five distinct OH⁻ bands located above 3600 cm⁻¹ at RT and seven narrow bands at 80 K and additional fine structure. The spectra were curve fit and the OH⁻ stretching modes analyzed and assigned. It is argued that OH⁻ is located in microscopic- and nano-size Ca₃Al₂H₁₂O₁₂-like clusters. The basic substitution mechanism is the hydrogarnet one, where (H₄O₄)⁴⁻ ⇔ (SiO₄)⁴⁻, and various local configurations containing different numbers of (H₄O₄)⁴⁻ groups define the cluster type. The amounts of H₂O range between 5 and 100 ppm by weight, depending on the IR calibration adopted, and are variable among crystals. Hydrogrossular-like clusters are also present in a synthetic pyrope with a minor Ca content grown hydrothermally at 900 °C and 20 kbar, as based on its IR spectra at RT and 80 K. Experiment and nature are in agreement, and OH⁻ groups are partitioned into various barely nano-size hydrogrossular-like clusters. This proposal is new and significant mineralogical, petrological, and geochemical implications result. Ca and proton ordering occur. Hypothetical “defect” and/or coupled-substitution mechanisms to account for structural OH⁻ are not needed to interpret experimental results. OH⁻ incorporation in pyrope of different generations at Dora-Maira is discussed and OH- could potentially be used as an indicator of changing P_(H₂O)(a_(H₂O)) – T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically.

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Geiger, Charles A.0000-0001-9212-354X
Rossman, George R.0000-0002-4571-6884
Additional Information:© The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit Received 09 December 2019; Accepted 29 April 2020; Published 02 June 2020. Open access funding provided by the Austrian Science Fund (FWF). We thank Christian Chopin (Paris), once again, for his donation of garnets from Dora Maira that we have used so often for different investigations and George Lager (Louisville—retired) for the synthetic hydrogrossular. Two anonymous reviewers suggested changes for improving the clarity of presentation. This research was supported by a grant from the Austrian Science Fund (FWF: P 30977-NBL) to C.A.G. and a NSF grant (EAR-1322082) to G.R.R. C.A.G. also thanks the “Land Salzburg” for financial support through the initiative “Wissenschafts- und Innovationsstrategie Salzburg 2025”.
Funding AgencyGrant Number
Fonds zur Förderung der wissenschaftlichen Forschung (FWF)P 30977-NBL
Subject Keywords:Pyrope; IR spectroscopy; H2O; Hydrogrossular nano clusters; High pressure; UHP metamorphism
Issue or Number:6
Record Number:CaltechAUTHORS:20200602-102816568
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Official Citation:Geiger, C.A., Rossman, G.R. Micro- and nano-size hydrogrossular-like clusters in pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps. Contrib Mineral Petrol 175, 57 (2020).
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103635
Deposited By: Tony Diaz
Deposited On:02 Jun 2020 17:44
Last Modified:16 Nov 2021 18:23

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