From Structure Topology To Chemical Composition. XXI. Understanding The Crystal Chemistry of Barium In TS-Block Minerals

There are 40 TS-block (Titanium–Silicate) minerals, which are divided into four groups, based on the topology and stereochemistry of the TS block. Each group of structures has a different linkage, content, and stereochemistry of Ti (+ Nb + Zr + Fe3+ + Mg + Mn). In the crystal structures of TS-block minerals, TS blocks either link directly or alternate with I (Intermediate) blocks. All TS-block structures are characterized by a planar cell based on minimal lengths of translational vectors t1 and t2, t1 ∼ 5.4 and t2 ∼ 7 Å, and t1 ^ t2 ≈ 90°. Barium is an I-block cation in 20 TS minerals. In the I block, Ba atoms form a close-packed layer. Ba occurs in Groups II–IV, but does not occur in Group I where TS-blocks link directly and an I block is absent. In TS-block structures, there are three types of I block containing Ba atoms. The type-1 I block is a layer of Ba atoms parallel to the TS block with [5]-coordinated Ti in the H sheets. The type-2 I block consists of a central layer of cation polyhedra and two adjacent layers of Ba associated with [5]- and [6]-coordinated Ti in the H sheets. For the crystal structures with I blocks of type-1 and type-2, the lengths of the t1 and t2 vectors are t1 ∼ 5.4 and t2 ∼ 7 Å. The type-3 I block consists of a layer of Ba or Ba (+ K + Na) atoms associated with [6]-coordinated Ti in the H sheets. The F and O atoms that are apical anions of [6]-coordinated Ti occur in the layer of Ba atoms. The crystal structure is based on two translation vectors, t1 and t2; the lengths of these vectors are doubled: 2t1 ∼ 10.8 and 2t2 ∼ 14 Å. In structures with the type-3 I block, doubled minimal translations are due to distortion of rings of polyhedra in the H sheets. This distortion of rings of polyhedra is necessary to satisfy bond-valence requirements on XPM anions, where XPM = F, O. In the type-3 I block, the XPM site is coordinated by four or six cations and there is no space to accommodate an OH group plus a hydrogen bond at that site. Hydroxyl groups occur at the XOA sites in the O sheet where each XOA anion is bonded to three MO cations. Hence F atoms are ordered at the XPM sites on the periphery of the TS block and OH groups are ordered at the XOA sites in the O sheet.