Abstract: Cation ordering in amesite-2H2 from Antarctica has reduced the true symmetry from the ideal hexagonal space group P63 to triclinic P1. All crystals show 6-fold biaxial twin sectors on (001), and the twinned crystals produce an average diffraction symmetry that is hexagonal. Individual twin sectors cut from the larger aggregate have 2V optic angles near 18°, slightly monoclinic unit-cell geometry, and triclinic diffraction symmetry. Structural refinement of an untwinned sector in subgroup symmetry shows nearly complete ordering of Si,Al in tetrahedral sites and of Mg,Al in octahedral sites.
In triclinic symmetry the two layers in the unit cell are no longer equivalent. Tetrahedra lying on the pseudo-63 screw axis are alternately Si-rich and Al-rich in adjacent layers. Of the three octahedral sites in each layer, one is smaller than the other two and is interpreted as Al-rich. The distribution of Al-rich and Mg-rich octahedra violates both the pseudo-3-fold rotation axis within each layer and the pseudo-63 screw axis that relates one layer to the next in the ideal space group. Local charge balance is achieved in adjacent layers by location of all tetrahedral and octahedral Al in lines parallel to X1 and spaced at intervals of b1/2. Similar charge balance patterns parallel to X2 and X3 are postulated to account for the sector twinning, which has been observed also in amesites from Chester, Massachusetts, USA, Saranovskoye, USSR, and Postmasburg, South Africa.