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
AbstractThe 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 Å. 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 anI(intermediate) block that comprises atoms between two TS blocks. The TS block propagates close packing of cations into theIblock. There are two types of close-packed layers of cations in theIblock: (I) a layer of Na+and P5+with mean cation–cation distances of 3.41 Å and (II) a layer of Ba2+(+K+, Sr2+and Na+) with mean cation–cation distances of 4.73 Å. The general topology of the TS block is independent of the topology and chemical composition of theIblock. However direct interaction between TS andIblocks takes place in the crystal structures of jinshajiangite, bobshannonite, bafertisite, hejtmanite, delindeite and cámaraite. Interaction of Ba atoms in theIblock 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 toC-centered.