TY - JOUR

T1 - Dissymmetrization of crystal structures of grossular-andradite garnets Ca3(Al, Fe)2(SiO4)3

AU - Frank-Kamenetskaya, O. V.

AU - Rozhdestvenskaya, I. V.

AU - Shtukenberg, A. G.

AU - Bannova, I. I.

AU - Skalkina, Yu A.

PY - 2007/8/1

Y1 - 2007/8/1

N2 - The crystal structures of three birefringent grossular-andradite natural garnets Ca3(Al,Fe)2(SiO4)3 were investigated using single-crystal X-ray diffraction data (MoKα, number of reflections measured 8065, 10619, 9213; R = 2.81, 2.74, 3.26%). According to the values of unit cell constants, inconsistent intensities of reflections and appearance of additional (forbidden) reflections explored garnets have different symmetry: cubic, sp. gr. $$Ia\overline 3 d$$ (Fe/(Fe + Al) = 0.078, Δn = 0.0002); orthorhombic, sp. gr. Fddd (Fe/(Fe + Al) = 0.58, Δn = 0.0089); triclinic, sp. gr. $$I\overline 1 $$ or I1 and pseudo-orthorhombic (Fe/(Fe + Al) = 0.23, Δn = 0.0066). Careful refinement of all crystal structures in space groups $$Ia\overline 3 d$$ , Fddd and $$I\overline 1 $$ has confirmed the symmetry reduction detected on the diffraction patterns and shown that dissymmetrization of cubic garnets connects with partial ordering of trivalent cations over Y-sites. Direct linear relationship between Fe-occupancy, an average Y-O bond lengths and octahedral O-O edges has been revealed. Cluster models of dissymmetrization have been regarded. Evidence for the "growth dissymmetrization" phenomena (kinetic phase transformations) as the reasons of the symmetry reduction of cubic garnets has been discussed. The reasonable assumption that the garnets crystal structures described as orthorhombic are triclinic, but the deviations from the orthorhombic symmetry so small, that cannot be manifested by of X-ray diffraction study has been taken.

AB - The crystal structures of three birefringent grossular-andradite natural garnets Ca3(Al,Fe)2(SiO4)3 were investigated using single-crystal X-ray diffraction data (MoKα, number of reflections measured 8065, 10619, 9213; R = 2.81, 2.74, 3.26%). According to the values of unit cell constants, inconsistent intensities of reflections and appearance of additional (forbidden) reflections explored garnets have different symmetry: cubic, sp. gr. $$Ia\overline 3 d$$ (Fe/(Fe + Al) = 0.078, Δn = 0.0002); orthorhombic, sp. gr. Fddd (Fe/(Fe + Al) = 0.58, Δn = 0.0089); triclinic, sp. gr. $$I\overline 1 $$ or I1 and pseudo-orthorhombic (Fe/(Fe + Al) = 0.23, Δn = 0.0066). Careful refinement of all crystal structures in space groups $$Ia\overline 3 d$$ , Fddd and $$I\overline 1 $$ has confirmed the symmetry reduction detected on the diffraction patterns and shown that dissymmetrization of cubic garnets connects with partial ordering of trivalent cations over Y-sites. Direct linear relationship between Fe-occupancy, an average Y-O bond lengths and octahedral O-O edges has been revealed. Cluster models of dissymmetrization have been regarded. Evidence for the "growth dissymmetrization" phenomena (kinetic phase transformations) as the reasons of the symmetry reduction of cubic garnets has been discussed. The reasonable assumption that the garnets crystal structures described as orthorhombic are triclinic, but the deviations from the orthorhombic symmetry so small, that cannot be manifested by of X-ray diffraction study has been taken.

KW - "anomalous" birefringence

KW - Crystal chemistry

KW - Dissymmetrization

KW - Grossular-andradite garnets

UR - http://www.scopus.com/inward/record.url?scp=34447501895&partnerID=8YFLogxK

U2 - 10.1007/s11224-007-9171-0

DO - 10.1007/s11224-007-9171-0

M3 - Article

AN - SCOPUS:34447501895

VL - 18

SP - 493

EP - 503

JO - Structural Chemistry

JF - Structural Chemistry

SN - 1040-0400

IS - 4

ER -