Redetermination of junitoite, CaZn₂Si₂O ₇·H₂O

The crystal structure of the mineral junitoite, ideally CaZn ₂Si₂O₇·H₂O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91-95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO ₄ tetrahedra parallel to the b axis, cross-linked by Si ₂O₇ tetrahedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca ²⁺ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H₂O molecule (site symmetry ..2) in a distorted octahedral coordination environment. However, some bond lengths, especially for the SiO₄ tetrahedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water molecule and a framework O atom.