On the Complex Zonality in Grandite Garnets and Implications

T. I. Ivanova, A. G. Shtukenberg, Yu. O. Punin, O. V. Frank-Kamenetskaya and P. B. Sokolov
Department of Crystallography, Saint-Petersburg State University, University Emb. 7/9, 199034 Saint-Petersburg, Russia

Abstract: The available data on compositional zoning in grossular-andradite (grandite) garnets occurring at different scales, mainly due to the variations of their Fe3+/Al ratios, and the hypotheses on the origin of this zoning are reviewed. Four zoned crystals of grandites showing different optical properties have been studied by means of X-ray diffraction. Optical and structural studies suggest three superimposed scales of Fe3+/Al zonality along [110]. The drastic change of the structural characteristics from sample to sample correlates with the change of their optical patterns. The superfine oscillatory zoning has been described in terms of irregular interstratified structures with a high degree of segregation. The self-affinity of zoning at different scales testify to the possible origin of the zonality due to the non-linear dynamics at the growth front. The wide variation of the layer compositions revealed by the modelling of the X-ray diffraction profiles seems to contradict the hypothesis of unmixing in grandite garnets.

Keyword: garnet • grandite • zoning

Mineralogical Magazine; December 1998 v. 62; no. 6; p. 857-868; DOI: 10.1180/002646198548089
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)