Refinement of hydrogen positions in natural chondrodite by powder neutron diffraction: implications for the stability of humite mineral

摘要

The structure of a natural sample of chondrodite (Mg_(4.89)Fe_(0.07)Si_(2.04)O_8F_(1.54)(OH)_(0.46)) was refined using powder neutron diffraction data and the Rietveld technique (P2_1/b; Z = 2; a = 4.7204(1) A; b = 10.2360(3) A; c = 7.8252(2) A; alpha = 109.11(1) deg; V = 357.26(2) A~3). Hydrogen was found to occupy the H1 site. The significance of hydrogen at this site is discussed in terms of hydrogen-bond stabilization of humite structures containing varying amounts of OH, F and Ti. Arguments are proposed as to why the F and Ti contents of natural humites usually result in only one H per formula unit when there is no crystal-chemical reason why fully hydrated samples should not be favoured.