The crystal chemistry of lamprophyllite-related minerals: a review
The crystal structures of the lamprophyllite-related minerals are based upon HOH modules consisting of a central octahedral O sheet sandwiched between two heteropolyhedral H sheets. The general crystal-chemical formula for these minerals can be written as [10-11]A2[ M1 M22  M3X 2] [ L 2(Si2O7)2O2], where the contents of the O and H sheets are given in square brackets in this order and A = Ba, Sr, K, Na; M1 = Na, Mn2+; M2 = Na, Mn2+, Fe2+, Ca; M3 = Ti, Mn2+, Mg, Fe3+, Fe2+; L = Ti, Fe3+; X = OH, O, F. According to the unit-cell parameters and symmetry, lamprophyllite-related minerals can be subdivided into five structure types: I (monoclinic polytypes, C2/m); II (orthorhombic polytypes, Pnmn), III (nabalamprophyllite, BaNa[Na3Ti(OH)2][Ti2(Si2O7)2O2], monoclinic, P2/m , with an ordered arrangement of the interlayer Ba2+ and Na+ cations), IV (triclinic, P1) and V (triclinic, P1). The triclinic members (types IV and V) include schüllerite and its analogues, which differ from the lamprophyllite-group minerals sensu stricto in their symmetry and topology of the HOH modules. The end-member formulae of lamprophyllite-related minerals and the position of schüllerite in the ranks of heterophyllosilicates are discussed.
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Document Type: Research Article
Publication date: December 31, 2016
This article was made available online on June 14, 2016 as a Fast Track article with title: "The crystal chemistry of lamprophyllite-related minerals".
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