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

Institute of Geoscience, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 305-8571, Japan

^* E-mail: kyono{at}esca.geo.tsukuba.ac.jp

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, ß = 116.11(1)°, V = 742.3(3) ų, 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 Rb_0.8110.127Al_1.059Si_3.003O₈. 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.

KEYWORDS: Rb-feldspar, Si₄O₈, Al-excess substitution, H₂O molecule

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