The derivative structure of silicate garnets in grandite

The crystal structures of birefringent silicate garnets have been studied using crystals from three different localities: North Moravia, Czechoslovakia, (Ca2 .9 6Fe0 0 4 ) (Al16oFeo.4o) (Si2.94Alo.oe) °i2> Munam, North Korea (Ca2.91Mn0.05Fe0.04) (Alj.33Fe0.65Ti0.02) (Si2.97Alo.03) 0 1 2 , and Kamaishi, Japan, (Ca2.98Mn0.02) (Alo^Fe , 1 6) (Si2.93Al0 0 7 ) 0 1 2 . 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 Fe3 + 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 Fe3 + 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 I41/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 54 Y . T a k e u c h i et a l . : T h e derivative s tructure o f silicate garnets in grandi tc order-disorder transition in silicate garnets appears to be strongly related to the difference in the degree of order at the octahedral positions. A variety of twinnings in silicate garnets can be studied in terms of mistakes that occur in the ordered array of the octahedral cations. Introduction Regarding optical anomaly of silicate garnets, X3Y2[Si04]3 (X = Mg, Fe + , Mn, Ca; Y = A1, Fe + , Cr + , V + ), we find in The System of Mineralogy (Dana, 1900) the description, "The anomalous double refraction of many garnets, early noted by Brewster, was fully studied by Mallard and later by Bertrand, by whom it has been explained as due to the complex twinning of triclinic individuals, producing at times forms which are apparently orthorhombic and isometric". Since the crystal structure of garnet was determined by Menzer (1926,1929), light has been thrown on the crystal-chemical features of garnets through a number of works such as those reported by Zemann (1962), Euler and Bruce (1965), Geller (1967) and Novak and Gibbs (1971). The existence of non-cubic derivatives of the garnet structure type were discovered by Aubry, Dusausoy, Laffaille and Protas (1969) for henritermierite, Ca 3 (Mn 1 5 Al 0 5 ) (Si04)2 (OH)4 and by Prewitt and Sleight (1968) for CdGeOj {or Cd3(Cd,Ge)[Ge04]3} and CaGe0 3 ; to the latter two, the high pressure form of MnSi0 3 (Akimoto and Syono, 1972) is thought to be isotypic. In spite of such a cumulation of the structural knowledge on garnets, the Bertrand's explanation on the optical anomaly of garnets has not yet been substantiated on the basis of crystal structure. It appears that the optical anomaly observed in cubic crystals in general tends to be ascribed to strain in the crystal rather than change in its crystal structure. The birefringence of diamond indeed has been well explained in terms of strain in crystal. Accoding to Lang (1967), the perfect diamond, free from strain, is an elusive ideal. For birefringent garnets of ugrandite, a series of the garnet group consisting of uvarovite, Ca3Cr2[Si04]3, grossularite, Ca3Al2[Si04]3, and andradite, Ca3Fe2[Si04]3 , Yorder (1950) however, suggested a possibility that the octahedral cations might be ordered, giving rise to a non-cubic derivative structure of the garnet structure type. This suggestion seems to be supported by a synthetic work of Kalinin (1967), who reported that the anisotropy in grandite, a member of the grossulariteandradite series, was related to its chemistry. Among other studies on optical anomaly of silicate garnets, one carried out by Isogami (1976) is noteworthy. Based on optical observations and their interpretation by means of elasticity theory, he concluded that optical anomalies of silicate garnets may be classified into two major types: the one which is commonly observed in grandite series is due to the kind of strain which is originated in the distortion of coordination polyhedra and so is structural; the other, which is Y . Takeuchi et a l . : The derivative structure o f silicate garnets in grandite 55 characteristic of pyralspite series, can be ascribed to strain due to impurities contained in the crystal. The X-ray study carried out by Takeuchi and Haga (1976) on a birefringent grandite garnet from Munam, North Korea, though of an exploratory nature, established that the structure was non-cubic in it the octahedral cations were partially ordered. The structure was very likely to be orthorhombic with possible space group Fddd. A result similar to this was also reported by Miyazaki, Kihara, Matsumoto and Sugiura (1976). The present paper presents the result of our further investigation of the Munam garnet, the study on two new birefringent garnets in grandite, and describes details of the non-cubic structure of the silicate garnets.