Localization of Small Impurities of Water and Carbon Dioxide in Channels of the Structure of Natural Cordierite

An accurate X-ray diffraction study of cordierite mineral, the simplified chemical formula of which is Mg2Al4Si5O18, is performed. The main octahedral position contains magnesium, iron, and aluminum ions, as well as traces of manganese. Alkaline cations are localized within large channels formed by six AlO4 and SiO4 tetrahedra at the level z = 0. Water and carbon-dioxide molecules are located within the same vertical channels as sodium ions but at the level z = 1/4; they alternate from one cell to another. The CO2 molecule is elongated along the a axis of the cell and rotates around the central carbon atom (0, 0, 0.25) by an angle of ~10°. Several orientations of the water molecule are observed. The mutual ordering of the molecules is hindered in the range 91–295 K due to intense interorientational transitions and the duration of residence of the water molecules in incoherent intermediate states.