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

Abstract

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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