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

Charles A Geiger1, George R Rossman2
Department Chemistry and Physics of Materials, Section
Materials Science and Mineralogy, Salzburg University,
Jakob Haringer Strasse 2a, A‑5020 Salzburg, Austria

Division of Geological and Planetary Science
California Institute of Technology
     Pasadena, CA  91125-2500, USA


The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultrahigh-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(αH2O) - T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically