Micro- and Nano-Size Hydrogarnet Clusters in Calcium Silicate Garnet: 

Part II. Mineralogical, Petrological and Geochemical Aspects


Charles A. Geiger1, George R. Rossman2

1Department of Chemistry and Physics of Materials, Salzburg University, A-5020 Salzburg Austria

2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125-2500, U.S.A.

ABSTRACT

The nominally anhydrous, calcium-silicate garnets, grossular - Ca3Al2Si3O12, andradite - Ca3Fe3+2Si3O12, schorlomite - Ca3Ti4+2(Si,Fe3+2)O12, and their solid solutions can incorporate various amounts of structural OH-. The IR single-crystal spectra of a number of calcium silicate garnets were recorded at room temperature between 3000 and 4000 cm-1. Spectroscopic results are also taken from the literature. The spectra are analyzed and show various OH- stretching modes with different wavenumbers between 3500 and 3700 cm-1. Following the conclusions of Part I of this study, the garnets appear to contain various microscopic- and nano-size Ca3Al2H12O12- and Ca3Fe3+2H12O12-like clusters dispersed throughout an anhydrous garnet “matrix”. Their sizes are roughly between 3 and 15 . The hydrogarnet cluster model can explain both IR spectra and also various other experimental results. Hypothetical “defect” mechanisms are not needed to account for OH- incorporation and behavior in garnet. Key, new understanding into published dehydration and H-species diffusion results, as well as H2O-concentration and IR absorption-coefficient determinations, is now possible for the first time at the atomic level. IR spectra with similar OH- bands are found between end-member synthetic and natural grossular crystals. Similar IR “OH-band patterns” are also found among different natural, calcium-silicate garnets possibly indicating that chemical equilibrium operated during their crystallization. Under this assumption, the hydrogarnet-cluster types and their concentrations could potentially be used to decipher petrologic (i.e., P-T-X) conditions under which a garnet crystal, and the rock in which it occurs, formed. Schorlomites from phonolites contain very minor amounts of H2O, whereas Ti-bearing andradites from low-grade chlorite schists can contain much more H2O. Different hydrogarnet clusters and concentrations can occur in metamorphic grossulars from the well-known locality at Asbestos, Quebec, Canada. IR absorption coefficients for H2O held in hydrogrossular- and hydroandradite-like clusters must be different in magnitude and this work lays out how they can be best determined. Hydrogen diffusion behavior in garnet crystals at high temperatures is governed by the temperature stability of the various local hydrogarnet clusters.

last revised 12 -Aug-2019