Refinement of the crystal structure of a synthetic non-stoichiometric Rb-feldspar

<jats:title>Abstract</jats:title><jats:p>The crystal structure of hydrothermally synthesized Rb-feldspar (monoclinic, space group <jats:italic>C</jats:italic>2/<jats:italic>m</jats:italic>, <jats:italic>a</jats:italic>= 8.839(2)Å, <jats:italic>b</jats:italic>= 13.035(2)Å, <jats:italic>c</jats:italic>= 7.175(2)Å, β = 116.11(1)8, <jats:italic>V</jats:italic>= 742.3(3)Å<jats:sup>3</jats:sup>, <jats:italic>Z</jats:italic>= 4) has been refined to a final <jats:italic>R</jats:italic> of 0.0574 for 692 independent X-ray reflections. Microprobe analyses of the Rb-feldspar suggest deviation from stoichiometry, with excess Si and Al, resulting in a unit formula of Rb<jats:sub>0.811</jats:sub>□<jats:sub>0.127</jats:sub>Al<jats:sub>1.059</jats:sub>Si<jats:sub>3.003</jats:sub>O<jats:sub>8</jats:sub>. Infrared (IR) spectra indicate the structural occupancy of large H<jats:sub>2</jats:sub>O content, which implies that the □Si<jats:sub>4</jats:sub>O<jats:sub>8</jats:sub> substitution favours the structural incorporation of the H<jats:sub>2</jats:sub>O molecule at the <jats:italic>M</jats:italic>-site. The mean <jats:italic>T</jats:italic>–<jats:italic>O</jats:italic> distances are 1.632 Å for <jats:italic>T</jats:italic>1 and 1.645 Å for <jats:italic>T</jats:italic>2, revealing highly disordered (Al,Si) distribution with Al/Si = 0.245/0.755 (<jats:italic>T</jats:italic>1 site) and 0.255/0.745 (<jats:italic>T</jats:italic>2 site).</jats:p><jats:p>There are two geochemical implications from this refinement: (1) identification of both rubicline triclinic with (Al,Si) ordered distribution and synthetic monoclinic RbAlSi<jats:sub>3</jats:sub>O<jats:sub>8</jats:sub> with (Al,Si) disordered distribution implies that Rb cannot be one of factors disrupting the (Al,Si) ordered and disordered distributions in feldspars; and (2) natural and synthetic feldspars capable of accommodating the large cations tend to incorporate □Si<jats:sub>4</jats:sub>O<jats:sub>8</jats:sub>, excess Al and H<jats:sub>2</jats:sub>O components in their crystal structures.</jats:p>