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 {Mg2.79,Fe2+0.15,Ca0.04}Σ2.98[Al]2.02(Si)2.99O12 and {Mg2.90,Fe2+0.04,Ca0.02}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 PH2O(aH2O) -T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically.