Nano-Size Hydrogarnet Clusters and Proton Ordering in Calcium Silicate Garnet: Part I. The Quest to Understand the Nature of "Water" in Garnet Continues.


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 and their solid solutions Ca3(Alx,Fe3+1-x)2Si3O12, can incorporate various amounts of structural OH-. This has important mineralogical, petrological, rheological and geochemical consequences and extensive experiment investigations have focused on the nature of “water” in these phases. However, it was not known how OH- was incorporated and this has seriously hampered the interpretation of many different research results. The IR single-crystal spectra of a number of calcium silicate garnets, both “end-member” and solid-solution compositions, were recorded at room temperature and 80 K between 3000 and 4000 cm-1. Five synthetic hydrogarnets in the system grossular-andradite-katoite (Ca3Al2H12O12)-Ca3Fe3+2H12O12 were also measured via IR ATR powder methods. The various spectra are rich in complexity and show a number of different wavenumber OH- stretching modes between 3500 and 3700 cm-1. The data, together with published results, were analyzed and modes assigned by introducing atomic-vibrational and crystal-chemical models to explain the energy of the OH- dipole and the structural incorporation mechanism of OH-, respectively. It is argued that OH- is located in various local microscopic- and nano-size Ca3Al2H12O12- and Ca3Fe3+2H12O12-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. Some spectra also possibly indicate the presence of tiny hydrous inclusion phases, as revealed by OH- modes at wavenumbers greater than about 3670 cm-1. They were not recognized in earlier studies. Published proposals invoking purely hypothetical “defect” and coupled-substitution mechanisms to account for OH- in garnet are not needed to interpret the IR spectra, at least for OH- modes above about 3560 cm-1. Significant mineralogical, petrological, geochemical and rheological implications result from the analysis and are discussed in Part II of the investigation.

last revised 12 -Aug-2019