The crystal chemistry of lamprophyllite-related minerals: a reviewстатья
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Аннотация: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)] A(2) [([6]) M1([6]) M2(2) ([6]) M3X(2)] [([5]) L-2(Si2O7)(2)O-2], 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)][Ti-2(Si2O7)(2)O-2], monoclinic, P2/m, with an ordered arrangement of the interlayer Ba2+ and Na+ cations), IV (triclinic, P1) and V (triclinic, P (1) over bar). The triclinic members (types IV and V) include schullerite 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 schullerite in the ranks of heterophyllosilicates are discussed.