The crystal structures of 2 M 1 phengite and 2 M 1 muscovite

Abstract The crystal structures of 2 M 1 phengite and 2 M 1 muscovite have been refined by least-squares methods using single-crystal diffractometer data. Final discrepancy factors were unweighted R = 0.045 for 2 M 1 phengite and 0.035 for 2 M 1 muscovite. Both specimens have the space group C 2/c and the following cell dimensions at room temperature: 2 M 1 phengite 2 M 1 muscovite a 5.2112 ± 0.0003 Å 5.1906 ± 0.0002 Å b 9.0383 ± 0.0004 9.0080 ± 0.0003 c 19.9473 ± 0.0006 20.0470 ± 0.0006 β 95.769 ± 0.005° 95.757 ± 0.002°. In 2 M 1 muscovite tetrahedral cation-oxygen distances T1–O = T2–O = 1.643 Å show that there is no ordering of (Si,Al) over the tetrahedral site. In the structure of 2 M 1 phengite, however, the corresponding values, T1–O = 1.622 and T2–O = 1.633 Å, indicate a very slight ordering. The framework of anions is distorted twice as much in 2 M 1 muscovite than in 2 M 1 phengite.