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

Abstract

AbstractThe structure of a natural sample of chondrodite (Mg4.89Fe0.07Si2.04O8F1.54(OH)0.46) was refined using powder neutron diffraction data and the Rietveld technique (P21/b; Z = 2; a = 4.7204(1)Å; b = 10.2360(3)Å; c = 7.8252(2)Å; α = 109.11(1)°; V = 357.26(2)Å3). 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.