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Abstract A monoclinic metastable phase of anorthite composition, CaAl 2 Si 2 O 8 , has been synthesized as an artificial product. Crystals are twinned mostly on a submicroscopic scale. In the cell whose dimensions are a = 8.228(1), b = 8.621(1), c = 4.827(1) Å, β = 90.00(5)°, there are two formula units. The space group is P 2 1 . The structure has been worked out with the minimum-function method based upon intensities measured on an automated diffractometer. Final atomic coordinates give an R of 0.06 with isotropic thermal parameters. The structure is a framework in which there can be recognized rings of four tetrahedra like the ones of felspar. The calcium atoms are statistically distributed in two point positions with the occupancies of 0.403(4) and 0.564(5) respectively. The structure has a pronounced pseudosymmetry of Pmnn . The deviation from this higher symmetry is primarily explained by nearly complete order in the Si and Al sites in the framework. A comparison of this structure with that of anorthite has revealed that the mean T–O–T angles at two- and three-coordinated oxygen atoms are smaller than the corresponding angles of anorthite. A mechanism of twinning is suggested in terms of mistakes in the Si and Al placements which may occur in the process of crystal growth. There is no evidence that the synthetic “orthorhombic CaAl 2 Si 2 O 8 ”, described by D avis and T uttle (1952), is distinct from the present monoclinic phase.
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