The structure of hydrous Li-, Cs-, and Na-rich beryl from the Harding, New Mexico, pegmatite has been refined to an R factor of 0.050 using single-crystal X-ray methods. Alkali ions plus water molecules were located by difference Fourier methods in two sites within the channels paralleling c. Orientation of water molecules was determined from polarized IR spectra. The resulting structural formula is
[W(I)0.09W(II)0.46Cs0.09K0.050.31]XII[Na0.21Ca0.02Mg0.02Fe0.013+0.74]VI[AI2.0]VI[Be2.59Li0.38Al0.02]IV[Si5.92Al0.08]IVO18,
where W(I) and W(II) represent Type I (H-H vector parallel to c) and Type II (H-H vector perpendicular to c) water, respectively, and □ represents vacancies. Li occupies the nonring tetrahedral site; water molecules, Cs, and K occupy the 12-coordinated channel site between six-membered silicate rings; and Na and Ca occupy the six-coordinated channel site within the rings.

The structure was also refined at 500, 800, and 24°C, after heating, and cell parameters were determined at 200, 300, 400, 500, 600, 700, and 800°C. The a and c cell parameters were found to expand over this temperature range with the following coefficients:ε1= 1.2 x 10-6 °C-1 and ε3 = 3.1-1.5 x 10-6 °C-1. The thermal expansion is consistent with changes in Gruneisen parameters, γ1, and γ3, with increasing temperature and can be rationalized in terms of (1) increases in volume of AlO6 octahedra and (Be,Li)O4 tetrahedra, (2) changes in T-O-Al and O-T-O angles (T = Si,Be,Li,Al), (3) rotation of (Be,Li)O4 tetrahedra about axes lying in (001), and (4) rotation of vertically adjacent six-membered rings in opposite senses about [001]. Our results contrast with the thermal expansion of nonalkali hydrous and anhydrous synthetic beryl, emerald, and low-alkali hydrous cordierite. Heat treatment at 800°C for 72 h had little effect on the occupancies of channel sites; little dehydration occurred because the large alkali ions effectively plug the channels.

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