The Crystal Chemistry of Kornerupine. II. The Role of Hydrogen


Mark A. Cooper and Frank C. Hawthorne
Department of Geological Sciences, University of Manitoba Winnipeg, Manitoba, Canada R3T 2N2
Luisa Ottolini
CNR-Centro di Studio per la Cristallochimica e la Cristallografia (CSCC)
via Abbiategrasso, 209, 1-27100 Pavia, Italy

Edward S. Grew
Department of Geological Sciences, University of Maine, Orono, ME 04469, U.S.A.

T. Kurtis Kyser
Department of Geological Sciences, Queen's University
Kingston, Ontario, Canada K7L 3N6
George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125 U.S.A.

ABSTRACT

 

    The H content of a suite of well-characterized samples of kornerupine-prismatine minerals, ([],Mg,Fe) (Al,Mg,Fe)9 (Si,Al,B)5 O21 (OH,F), has been measured by KFT (Karl Fischer Titration) and HLE (Hydrogen-Line Extraction). The KFT results show (OH + F) < 1.0 apfu (atoms per formula unit) and the deviation from 1.0 (OH + F) apfu is negatively correlated withthe Fe content of kornerupine, whereas the HLE results show (OH + F) = 1.0 apfu. Crystalchemical considerations suggest that the HLE results are correct. The anomalously low KFT results are due to incomplete breakdown of the sample and only partial release of the constituent H. Detailed examination of the electron density around the O(10) (= OH) site in several kornerupine crystals showed two distinct H sites, H(1) and H(2); the H(1) site occurs onthe (001) mirror plane, and the H(2) site occurs off the (001) mirror plane. The H atom is usually disordered off the (001) mirror plane (at z = 1/4) at the H(2) site. The latter is only ~1.1 Å from the X site, and hence mutual occupancy of locally adjacent H(2) and X sites is not possible; this constraint gives the maximum possible occupancy of the X site as (1 + F)/2 apfu. When H occupies the H(1) site on the (001) mirror plane, both adjacent X sites must be locally vacant. Polarized infrared spectra in the principal (OH)-stretching region show fine structure that can be assigned to specific short-range arrangements of cations around the O(10) [= (OH), F] site involving occupancy of the X site by divalent cations or vacancies, adjacent T(1) sites by Si–Si or Si–Al, adjacent M(1) sites by Mg or Fe2+, and occupancy of the H(1) or H(2) sites by H or [].

 Structure

 Canadian Mineralogist: 47, 263-274.