The derivative structure of silicate garnets in grandite

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 |

Structural dissymmetrization of optically anisotropic Grs64±1Adr36±1Sps2grandite from Meka Presedla (Kopaonik Mt., Serbia)

“…And last two chosen criterions are R B and R F reliability factors, whose values should be at least better than those obtained for the Ia 3d space group (Supplementary Material 6, third row). Namely, distances, sofs, and lower comparative R-values were most of the decisive factors for the space group definition in vast majority of the previously studied low symmetry ugrandites (Takéuchi et al, 1982;Allen and Buseck, 1988;Wildner and Andrut, 2001;Shtukenberg et al, 2005;Frank-Kamenetskaya et al, 2007). Beside these, for accessory criterions, there were chosen their estimated standard deviations (e.s.d.s).…”

“…phase transition from the Ia 3d to the R 3c, Fddd, C2/c, and I 1 space groups (Hatch and Griffen, 1989) and also in conformity with our previous results. However, because some confusion arises due to the different monoclinic settings in previous papers (Takéuchi et al, 1982;Gali, 1983;Hatch and Griffen, 1989;Wildner and Andrut, 2001), we shall also include the I2/c space group for further reconsideration too. Therefore, the remaining solutions that should be considered are R 3c, Fddd, I2/c, and C2/c space groups.…”

“…A substantial number of previously observed grandites exhibit optical anisotropy, indicating five possible causes for this phenomenon, such as (1) twinning (Ingerson and Barksdale, 1943;Brown and Mason, 1994); (2) substitution of rare-earth elements (REE) for Ca (Blanc and Maisonneuve, 1973); (3) non-cubic distribution of OH − groups, due to hydrogarnet substitution (Aines and Rossman, 1984;Rossman and Aines, 1986;Allen and Buseck, 1988;Andrut et al, 2002;Badar et al, 2016); (4) residual strain from lattice mismatch at compositional, twin, or grain boundaries (Chase and Lefever, 1960;Lessing and Standish, 1973;Foord and Mills, 1978;Kitamura and Komatsu, 1978;Hofmeister, 1993, 1995;Hofmeister et al, 1998;Shtukenberg et al, 2001;Antao, 2013aAntao, , 2013bAntao and Klincker, 2013); and 5ordering of Fe 3+ and Al 3+ over the octahedral sites (Takéuchi and Haga, 1976;Takéuchi et al, 1982;Gali, 1983Gali, , 1984Akizuki, 1984Akizuki, , 1989Allen and Buseck, 1988;Hatch and Griffen, 1989;Kingma and Downs, 1989;Griffen et al, 1992;Akizuki et al, 1998;Shtukenberg et al, 2001Shtukenberg et al, , 2002Shtukenberg et al, , 2005Shtukenberg et al, , 2006Frank-Kamenetskaya et al, 2007;Kobayashi et al, 2013;…”

Optical Anomaly in Iridescent Andradite From the Sierra Madre Mountains, Sonora, Mexico

“…review of the literature Brauns (1891) summarized optical studies of birefringent sector-zoned crystals of garnet. Takéuchi et al (1982) found that Al 3+ and Fe 3+ ions are distributed at the octahedral sites in an ordered manner. The birefringence was attributed to a lowering of the symmetry of garnet to orthorhombic (Fddd) or triclinic (I1).…”

“…The birefringence was attributed to a lowering of the symmetry of garnet to orthorhombic (Fddd) or triclinic (I1). Rossman & Aines (1986) studied the sample of Takéuchi et al (1982) by FTIR and found that the distribution of OH is isotropic. Allen & Buseck (1988) described a non-isotropic distribution of OH groups in anisotropic garnets.…”