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Micro- and nano-size hydrogarnet clusters in calcium silicate garnet: Part II. Mineralogical, petrological, and geochemical aspects

Geiger, Charles A. and Rossman, George R. (2020) Micro- and nano-size hydrogarnet clusters in calcium silicate garnet: Part II. Mineralogical, petrological, and geochemical aspects. American Mineralogist, 105 (4). pp. 468-478. ISSN 0003-004X. doi:10.2138/am-2020-7257.

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The nominally anhydrous, calcium-silicate garnets, grossular (Ca₃Al₂Si₃O₁₂), andradite (Ca₃Fe³⁺₂Si₃O₁₂), schorlomite (Ca₃Ti⁴⁺₂ [Si,Fe³3⁺₂]O₁₂), and their solid solutions can incorporate structural OH⁻, often termed “water.” The IR single-crystal spectra of several calcium silicate garnets were recorded between 3000 and 4000 cm ⁻¹. Spectroscopic results are also taken from the literature. All spectra show various OH⁻ stretching modes between 3500 and 3700 cm⁻¹ and they are analyzed. Following the conclusions of Part I of this study, the garnets appear to contain local microscopic- and nano-size Ca₃Al₂H₁₂O₁₂₋ and Ca₃Fe³⁺₂H₁₂O₁₂-like domains and/or clusters dispersed throughout an anhydrous “matrix.” The 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. A single (H₄O₄) group is roughly 3 Å across and most (H₄O₄)-clusters are between this and 15 Å in size. This model can explain the IR spectra and also other experimental results. Various hypothetical “defect” and cation substitutional mechanisms are not needed to account for OH⁻ incorporation and behavior in garnet. New understanding at the atomic level into published dehydration and H-species diffusion results, as well as H₂O-concentration and IR absorption-coefficient determinations, is now possible for the first time. End-member synthetic and natural grossular crystals can show similar OH⁻ “band patterns,” as can different natural garnets, indicating that chemical equilibrium could have operated during their crystallization. Under this assumption, the hydrogarnet-cluster types and their concentrations can potentially be used to decipher petrologic (i.e., P-T-X) conditions under which a garnet crystal, and the rock in which it occurs, formed. Schorlomites from phonolites contain no or very minor amounts of H₂O (0.0 to 0.02 wt%), whereas Ti-bearing andradites from chlorite schists can contain more H₂O (∼0.3 wt%). Different hydrogarnet clusters and concentrations can occur in metamorphic grossulars from Asbestos, Quebec, Canada. IR absorption coefficients for H₂O held in hydrogrossularand hydroandradite-like clusters should be different in magnitude and this work lays out how they can be best determined. Hydrogen diffusion behavior in garnet crystals at high temperatures is primarily governed by the thermal stability of the different local hydrogarnet clusters at 1 atm.

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Rossman, George R.0000-0002-4571-6884
Additional Information:© 2020 Mineralogical Society of America. Manuscript received August 12, 2019; Manuscript accepted November 20, 2019; Manuscript handled by Roland Stalder. E. Libowitzky (Vienna) kindly provided IR data for the Ti-bearing garnets from the study of Armbruster et al. (1998). This research was supported by grants from the Austrian Science Fund (FWF: P 30977-NBL) to C.A.G. and the NSF (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.” H. Skogby (Stockholm) and K. Wright (Perth) made constructive comments that improved the manuscript.
Funding AgencyGrant Number
FWF Der WissenschaftsfondsP 30977-NBL
Wissenschafts- und Innovationsstrategie Salzburg2025
Subject Keywords:Andradite, grossular, schorlomite, nominally anhydrous minerals, hydrogarnet clusters, IR spectroscopy, H2O, metamorphism, Water in Nominally Hydrous and Anhydrous Minerals
Issue or Number:4
Record Number:CaltechAUTHORS:20200401-124707417
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Official Citation:Charles A. Geiger, George R. Rossman; Micro- and nano-size hydrogarnet clusters in calcium silicate garnet: Part II. Mineralogical, petrological, and geochemical aspects. American Mineralogist ; 105 (4): 468–478. doi:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102241
Deposited By: Tony Diaz
Deposited On:01 Apr 2020 20:24
Last Modified:16 Nov 2021 18:10

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