The M 2 (PO 4 ) 2 Φ 8 cyclic tetramer – a flexible structure-building unit in phosphate minerals. II. Triphylite hydrothermal and supergene alteration products.

The triphylite-lithiophilite series, Li(Fe 2+ ,Mn 2+ )PO 4 , of primary phosphate minerals are notable for their susceptibility to alteration and their extensive range of alteration products.This article considers the principal crystallographic motifs that relate the crystal structures of the alteration minerals and their relationships to the parent structures.For alteration under oxidizing conditions the common structural motif is a laueite-like heteropolyhedral layer comprising 7 corner-connected octahedral chains, cross-linked by M 2 (TO 4 ) 2 8 cyclic tetramers.With decreasing temperature of hydrothermal alteration, the structures change, from containing two sets of interpenetrating, quasi-orthogonal, laueite-type layers, to containing one orientation of the laueite-type layers, in two-layer-wide slabs.Alteration minerals formed under supergene conditions, such as the laueite-group minerals have single laueite-type layers interconnected via corner-sharing with hydrated octahedrally coordinated divalent cations.Under reducing conditions, the alteration phases are hydrated minerals, which have structures based on strings of edge-sharing octahedra.They have, in common with the oxidized alteration products, the linking of the octahedral chains by cyclic tetramers that polymerize as krhnkite-related chains.Motifs present in the structures of the alteration phases can also be identified in the parent structures, suggesting that they may be nucleating sites for the formation/growth of the alteration phases.