No abstract

No abstract

The crystal structures of birefringent silicate garnets have been studied using crystals from three different localities: North Moravia, Czechoslovakia, (Ca2.96Fe0 04) (Al16oFeo.4o) (Si2.94Alo.oe) °i2> Munam, North Korea (Ca2.91Mn0.05Fe0.04) (Alj.33Fe0.65Ti0.02) (Si2.97Alo.03) 012, and Kamaishi, Japan, (Ca2.98Mn0.02) (Alo^Fe, 16) (Si2.93Al0 07) 012. Cell dimensions show salient features which include (1) one of the axial angles, of which we chose β, is significantly deviated from 90°: 90.07(1)° for Moravia garnet; 90.12(1)° for Munam garnet; and 90.07(1)° for Kamaishi garnet, and(2) the a and c lengths are equal within experimental error. These situations permit using an orthorhombic cell, a', b', c', with a' = a + c, b' = b, and c' = c -a. The diffraction symmetry is basically mmm with the orientation of the set of mirror planes consistent with the orthorhombic cell. That of Munam garnet, however, appears to be Τ rather than mmm.Difference Fourier syntheses, which were calculated using the symmetry relation of /T, revealed eight positive and eight negative residual peaks at the octahedral positions in the pseudocubic cell; their relative arrangements are the same throughout the three cases. The difference in absolute peak height between the residual peaks is negligible in the cases of Moravia and Kamaishi garnets, the structures being consistent with Fddd. The contents of Fe 3 + at the two non-equivalent octahedral sites of the Fddd structures are: 25.5(4) % and 14.5(4)% for Moravia garnet, and 65(2)% and 51(2)% for Kamaishi garnet. The difference in the Munam case is not negligible, yielding the structure which is consistent with the space group / I; the content of Fe 3 + in the eight non-equivalent octahedral positions varies from 15(1) % to 49(1) %.There is a simple relation between the above mode of cation ordering in the silicate garnets and that in the I4 1 /a structure type. If the difference in ionic radius between two kinds of octahedral cations is large and they undergo an ordered arrangement, the Mj/a type seems to be favored than the Fddd (or /T) type. The reported difference in energy which is required for the Brought to you by |