Crystal chemistry of natural layered double hydroxides; 2, Quintinite-1M; first evidence of a monoclinic polytype in M (super 2+) -M (super 3+) layered double hydroxides
Crystal chemistry of natural layered double hydroxides; 2, Quintinite-1M; first evidence of a monoclinic polytype in M (super 2+) -M (super 3+) layered double hydroxides
Mineralogical Magazine (October 2010) 74 (5): 833-840
Quintinite-1M, [Mg (sub 4) Al (sub 2) (OH) (sub 12) ](CO (sub 3) )(H (sub 2) O) (sub 3) , is the first monoclinic representative of both synthetic and natural layered double hydroxides (LDHs) based on octahedrally coordinated di- and trivalent metal cations. It occurs in hydrothermal veins in the Kovdor alkaline massif, Kola peninsula, Russia. The structure was solved by direct methods and refined to R (sub 1) =0.031 on the basis of 304 unique reflections. It is monoclinic, space group C2/m, a=5.266(2), b=9.114(2), c=7.766(3) Aa, beta =103.17(3) degrees , V=362.9(2) Aa (super 3) . The diffraction pattern of quintinite-1M contains sharp reflections corresponding to the layer stacking sequence characteristic of the 3R rhombohedral polytype, and rows of weak superlattice reflections superimposed upon a background of streaks of modulated diffuse intensity parallel to c (super *) . These superlattice reflections indicate the formation of a 2-D superstructure due to Mg-Al ordering. The structure consists of ordered metal hydroxide layers and a disordered interlayer. As the unit cell contains exactly one layer, the polytype nomenclature dictates that the mineral be called quintinite-1M. The complete layer stacking sequence can be described as... =Ac (sub 1) B=Ba (sub 1) C=Cb (sub 1) A=... Quintinite-1M is isostructural with the monoclinic polytype of [Li (sub 2) Al (sub 4) (OH) (sub 12) ](CO (sub 3) )(H (sub 2) O) (sub 3) .