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The crystal structure of anorthite, CaAl 2 Si 2 O 8  (Val di Fassa, Monzoni, Italy, An, 100%) has been refined by full-matrix least-squares methods using single-crystal neutron diffraction data (sin θ / λ  ≤ 0.78 Å −1 ) measured at 514 K. Crystal data: triclinic, space group  I [unk] (assumed),  Z  = 8,  a  = 8.194(1),  b  = 12.878(4),  c  = 14.171(6) Å;  α  = 93.07(2),  β  = 115.07(2),  γ  = 91.47(2)°,  V  = 1343.6 Å 3 ,  μ n   = 4.2 × 10 −3  cm −1  F (000) = 532.16 fm,  λ  = 1.0462(1) Å.  R ( F  2 ) = 0.049,  wR ( F  2 ) = 0.068,  S  = 1.27 for 5098 reflections and 293 parameters, including 40 third-order thermal tensor coefficients for four oxygen atoms. The aluminosilicate framework is virtually body-centered, whereas the Ca atoms occur in split-positions with unequal site occupancies. The eight tetrahedral sites show complete Al/Si ordering based on refined scattering-lengths values. The 16 distinct Si–O and [Al–O] bonds (e.s.d.'s, 0.001 Å) have average values of 1.617 ± 0.031 Å and [1.741 ± 0.029 Å]. The O–Si–O and [O–Al–O] angles are in the ranges 101.44(6) – 117.60(7)° and [97.87(7)–119.01(7)°] and have averages of 109.42 and [109.31°]. The unique Si–O–Al angles are between extremes 124.16(6) and 154.7(2)°. The two non-equivalent Ca atoms occupy statistically two sites with respective separations, 0.834(5), 0.494(5) Å, and weights equal ⅔, ⅓ within limits of the analysis. These results are interpreted in terms of a dynamical model in which the  I [unk] phase is a statistical dynamical average of small mobile  P [unk] domains. The unequal Ca occupancy in the split positions results from the persistence of considerable short-range order near  T c  . Significant anharmonic thermal vibrations of some of the oxygen atoms are attributed to critical fluctuations associated with the phase transition near  T c  .

zeitschrift für kristallographie - crystalline materials

Neutron diffraction studies of the  P [unk] →  I [unk] transition in anorthite, CaAl 2 Si 2 O 8 , and the crystal structure of the body-centered phase at 514 K