Growth ordering and anomalous birefringence in ugrandite garnets

摘要

Intermediate members of the grossular–andradite and grossular–uvarovite series are known to display anomalous birefringence, which is inconsistent with the ideal cubic space group of garnet Iad. To determine the reason for such birefringence, the crystal structures of at least 15 samples were refined by different authors including the present ones. The crystals with the value of anomalous birefringence of n > 0.001 are normally characterized by partial ordering of the octahedral cations (Al/Fe and Al/Cr for grossular–andradite and grossular–uvarovite series, respectively). This reduces the symmetry to the orthorhombic space group Fddd or even to the triclinic space group I. As is seen from the distribution of occupancies over octahedral sites in the triclinic space group, eight occupancies are grouped into two quartets of similar occupancies, leading to pseudo-orthorhombic crystal structures. The variety of structures with different degrees of pseudo-orthorhombicity is due to the action of the growth dissymmetrization mechanism. Simulations of the optical indicatrix in the point-dipole approximation confirm cation ordering as the main cause of the anomalous birefringence.