A 3T lepidolite with composition near K2(Al2.5R0.2Li3.2)(Si7Al)O20(F3OH), a = 5.200, с = 29.76A, space group Р3112, has been refined from diffractometer data using least-squares methods. Octahedral cations are ordered on three sites: M1 has (Li0.7A10.3), М2 has (Li0.9Al0.1), and М3 has (Li0.1Al0.9). M1 is the trans octahedron. Significant bond-length differences also indicate tetrahedral ordering for the two independent tetrahedra. T1 has (Al0.2Si0.8), and T2 has Si only. The σ and Δ values of 7.7° and 0.36A due to tetrahedral rotation conform to the empirical relation of McCauley and Newnham (1971).

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