Crystallographic Analysis of the Datolite (CaBSiO4OH) to Okaymalite (Ca2B2SiO7) Structural Transformation under Dehydration

The common “template” of datolite and okayamalite structures is a column-ribbon formed by alternating empty and Si occupied oxygen tetrahedra interconnected by common edges. The columns consolidate with their environment into crystalline structures largely due to the influence of the local symmetry of SiO4 and BO4 tetrahedra. As a result, mirror symmetry pseudo-planes of monoclinic datolite transform into okayamalite m planes; the pseudo-axis $\bar 4$ becomes the principal axis, whereas both structures contain glide planes and cation sublattices close in size.