The crystal structure of hydrothermally synthesized Rb-feldspar (monoclinic, space group C2/m, a = 8.839(2) Å, b = 13.035(2) Å, c = 7.175(2) Å, β = 116.11(1)°, V = 742.3(3) Å3, Z = 4) has been refined to a final R 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 Rb0.811□0.127Al1.059Si3.003O8. Infrared (IR) spectra indicate the structural occupancy of large H2O content, which implies that the □Si4O8 substitution favours the structural incorporation of the H2O molecule at the M-site. The mean T–O distances are 1.632 Åfor T1 and 1.645 Åfor T2, revealing highly disordered (Al,Si) distribution with Al/Si = 0.245/0.755 (T1 site) and 0.255/0.745 (T2 site).
There are two geochemical implications from this refinement: (1) identification of both rubicline triclinic with (Al,Si) ordered distribution and synthetic monoclinic RbAlSi3O8 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 □Si4O8, excess Al and H2O components in their crystal structures.