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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 Si 2 O 7  groups 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+Fe 3+ +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 P 5+  with mean cation–cation distances of 3.41 Å and (II) a layer of Ba 2+  (+K + , Sr 2+  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 the  I  block. However direct interaction between TS and  I  blocks takes place in the crystal structures of jinshajiangite, bobshannonite, bafertisite, hejtmanite, delindeite and cámaraite. Interaction of Ba atoms in the  I  block and F (+O) atoms of the TS block results in doubling of the minimal translations, 2 t  1  and 2 t  2 , and a concomitant change in symmetry of the structure from primitive to  C -centered.

zeitschrift für kristallographie - crystalline materials 

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