Abstract
Several hydrous silicates with similar chemical formulae and related crystal structures form under low-grade metamorphism and hydrothermal activity. Among them, epidote and pumpellyite are the most important minerals because of their common occurrence. Moreover, sursassite and iso-structural macfallite were also categorized as structurally related to pumpellyite. Their cation distributions and structural features are similar to those in epidote and pumpellyite. The following subjects are reviewed: 1) the topological relation of crystal structures among these minerals; 2) The cation distributions among the octahedral sites in epidote, pumpellyite, sursassite and macfallite and their effect on the structural variations; 3) the relationship between the oxidation state of transition elements and the hydrogen bonding systems in these hydrous silicates. Special attention was paid to the variety of hydrogen-bond systems with the oxidation states of transition elements, Me2+ + OH- ↔ Me3+ + O2-, commonly occurring in the pumpellyite, sursassite and macfallite structures. A model of the structural relationship among pumpellyite, sursassite, macfallite and epidote is proposed from a new stand point.
