On the complex zonality in grandite garnets and implications

The available data on compositional zoning in grossular-andradite (grandite ) garnets occurring at different scales, mainly due to the variations of th eir Fe3+/Al ratios, and the hypotheses on the origin of this zoning are rev iewed. Four zoned crystals of grandites showing different optical propertie s have been studied by means of X-ray diffraction. Optical and structural s tudies suggest three superimposed scales of Fe3+/Al zonality along [110]. T he drastic change of the structural characteristics from sample to sample c orrelates with the change of their optical patterns. The superfine oscillat ory zoning has been described in terms of irregular interstratified structu res with a high degree of segregation. The self-affinity of zoning at diffe rent scales testify to the possible origin of the zonality due to the non-l inear dynamics at the growth front. The wide variation of the layer composi tions revealed by the modelling of the X-ray diffraction profiles seems to contradict the hypothesis of unmixing in grandite garnets.