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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, Mg 3 Al 2 Si 3 O 12 , 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 ,Fe 2+  0.15 ,Ca 0.04 } Σ2.98 [Al] 2.02 (Si) 2.99 O 12  and {Mg 2.90 ,Fe 2+  0.04 ,Ca 0.02 } Σ2.96 [Al] 2.03 (Si) 3 O 12  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 Ca 3 Al 2 H 12 O 12 -like clusters. The basic substitution mechanism is the hydrogarnet one, where (H 4 O 4 ) 4-  ⇔ (SiO 4 ) 4- , and various local configurations containing different numbers of (H 4 O 4 ) 4-  groups define the cluster type. The amounts of H 2 O 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    $$ P_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{(}}a_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{) - }}T $$  P H 2 O ( a H 2 O ) - T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically.

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