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

Abstract

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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