The Crystal Chemistry of the Kornerupine–Prismatine Series. II. The Role of Hydrogen

Abstract

The H contents of a suite of well-characterized samples of kornerupine–prismatine minerals, (□,Mg,Fe) (Al,Mg,Fe)_9 (Si,Al,B)_5 O_(21) (OH,F), have 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); the deviation from 1.0 (OH + F) apfu is negatively correlated with the Fe content of kornerupine, whereas the HLE results show (OH + F) ≈ 1.0 apfu. Crystal-chemical 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 crystals of kornerupine reveal two distinct H sites, H(1) and H(2); the H(1) site occurs on the (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 = ¼) 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. Where 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 Fe^(2+), and occupancy of the H(1) or H(2) sites by H or □.

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