Structures with finite clusters of tetrahedra and octahedra

Structures with finite clusters of tetrahedra and octahedra 

Phosphate minerals of this class are listed in Table 5. 

Structures with infiuite chains of (PO4) tetrahedra and (M ) octahedra 

Jahnsite, CaMn Mg2[Fe (P04)2(OH)](H20)8, consists of [Fe ijis] chains of /ram-corner-sharing octahedra that are decorated by bridging (PO4) groups to give the general form [M (T04)2(t)] (Fig- l d). These chains extend in the Zi-direction and have a repeat distance of -7.1 A, leading Moore (1970) to designate these, and related, chains as the 

7-A chains. These chains are linked in the a-direction by [6]-coordinated Ca (Fig. 20c) that form chains of edge-sharing polyhedra in the -direction, forming slabs (Fig. 20d) parallel to (100) that are linked by octahedrally coordinated divalent-metal cations and by hydrogen bonding. In addition to the minerals of this group listed in Table 6, Matsubara (2000) reports the Fe equivalent of jahnsite, ideally CaFe e 2[Fe (P04)2(0H)]2 (H20)g, but this has not been approved as a valid species by the IMA. 

---