Abstract
In the Hakone district, there are two types of anorthite megacryst which are coloured yellow and colourless. Preliminary analysis by XPS had shown that iron in the yellow megacryst was ferric. Crystal structures of the above yellow and colourless anorthite megacrysts were refined from single crystal
diffraction data in space group P-1 and I-1 respectively. Average T-O bond distances for both megacrysts suggested that Al/Si distributions in the T-site were highly ordered. In particular, the T1(0z00)-site in the
yellow sample, the T-O bond distance was slightly longer and the isotropic temperature factor was just smaller than the other Al-sites. Judging from the compositional data (EPMA) and the structural evidence (XRD) it seems reasonable for ferric iron to enter T-site in the yellow one, and for ferrous iron into
the M-site of the colourless sample. These results imply that a somewhat large and heavy cation occupies the T1(0z00)-site, namely, partial replacement of Al3+ by Fe3+. Thus, the colouration in the yellow anorthite megacryst was confirmed structurally.
