From structure topology to chemical composition. XXXII. Titanium silicates of the seidozerite supergroup: crystal chemistry of Nb

The TS-block (Titanium-Silicate) is the main structural unit for the seidozerite-supergroup minerals.It consists of a central O (Octahedral) and two adjacent H (Heteropolyhedral) sheets where Si 2 O 7 groups occur.Cations in each sheet of the TS block are arranged as a closepacked layer forming an ABC close-packing.The fifty-two TS-block minerals are divided into four groups, based on the content and stereochemistry of Ti (+ Nb + Zr + Fe 3+ + Mg + Mn) and topology defined by the linkage of H and O sheets.In rinkite, bafertisite, lamprophyllite and murmanite groups, Ti (+ Nb + Zr + Fe 3+ + Mg + Mn) equals 1, 2, 3 and 4 apfu.In the crystal structures of TS-block minerals, TS blocks either link directly or alternate with I (Intermediate) blocks.As a dominant cation at a specific site, [6] Ti is substituted by Nb 5+ (12 minerals), Zr 4+ (2 minerals), Mg 2+ (2 minerals), Mn 2+ (1 mineral) and Fe 3+ (1 mineral).The crystal chemistry of Nb 5+ has been considered in the rinkite, bafertisite and lamprophyllite groups where Nb 5+ occurs as a dominant cation at the M O 1 site in the O sheet or at the M H site in the H sheet. Nb-dominant mineral species do not occur in the murmanite group.The different types of linkages in minerals with the TS block are a result of the bond-valence requirements of anions X O M and X O A shared by the O and H sheets, excluding the anions of the Si 2 O 7 group.The stereochemistry of the [4]-coordinated X O M anion in the O sheet, which is a bridge anion for the M H cation in the H sheet and three M O cations in the O sheet, has been considered.New developments, such as existence of counterpart structures in lamprophyllite and murmanite groups and successful prediction of three new minerals, have been reported.