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
1 Department
Chemistry and Physics of Materials, Section
Materials Science and Mineralogy, Salzburg University,
Jakob Haringer Strasse 2a, A‑5020 Salzburg, Austria
2 Division
of Geological
and Planetary
Science
California
Institute of
Technology
Pasadena,
CA 91125-2500, USA
ABSTRACT
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