From structure topology to chemical composition. XXV: New insights into the close packing of cations in the structures of the seidozerite-supergroup TS-block minerals

The titanium-silicate (TS) block is the main structural unit in the 45 seidozerite-supergroup minerals; it consists of a central O (O=Octahedral) sheet and two adjacent H (H=Heteropolyhedral) sheets where Si2O7groups occur in the H sheets. The three HOH sheets of the TS block form a three-layered close packing of cations with an ABC repeat; mean cation-cation distances are 3.41 \uc5. Minerals of the seidozerite supergroup are divided into four groups based on the content of Ti and topology and stereochemistry of the TS block: in rinkite, bafertisite, lamprophyllite and murmanite groups, Ti (+Nb+Zr+Fe3++Mg+Mn)=1, 2, 3 and 4 apfu, respectively. All TS-block structures consist either solely of TS blocks or of two types of block: the TS block and an I (intermediate) block that comprises atoms between two TS blocks. The TS block propagates close packing of cations into the I block. There are two types of close-packed layers of cations in the I block: (I) a layer of Na+and P5+with mean cation-cation distances of 3.41 \uc5 and (II) a layer of Ba2+(+K+, Sr2+and Na+) with mean cation-cation distances of 4.73 \uc5. The general topology of the TS block is independent of the topology and chemical composition of the I block. However direct interaction between TS and I blocks takes place in the crystal structures of jinshajiangite, bobshannonite, bafertisite, hejtmanite, delindeite and c\ue1maraite. Interaction of Ba atoms in the I block and F (+O) atoms of the TS block results in doubling of the minimal translations, 2t1and 2t2, and a concomitant change in symmetry of the structure from primitive to C-centered