Original paper

Crystal structure of Pb6[Ge6O18]·2H2O, a lead cyclo-germanate similar to the mineral dioptase

Hermann Otto, Hans; Brandt, Hans-Jürgen

European Journal of Mineralogy Volume 8 Number 2 (1996), p. 301 - 310

29 references

published: Apr 26, 1996
manuscript accepted: Nov 15, 1995
manuscript received: Mar 9, 1995

DOI: 10.1127/ejm/8/2/0301

BibTeX file

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Abstract Pb6[Ge6O18]·2H2O crystallizes as transparent and colourless trigonal needles, having space group R3̄, lattice parameters a = 15.6476(7) and c = 7.3119(4) Å, with Z = 3 formula units per unit cell. The density of 6.44 gem-3 is comparatively low. Powder X-ray diffraction data lead to a crystal structure yielding a final residual of RF = 0.033 (wR2 = 0.070, Rp = 0.106, Rwp = 0.133) after least squares refinement. The structure consists of sandwiched hetero-polyhedral rings with straight channels down [001]. Isolated and partly expanded rings of six corner-sharing GeO4 tetrahedra are bonded together vertically by crown-shaped Sechser rings of flat PbO5+1 pyramids with shared corners, which themselves share edges laterally. Lone electron pairs of lead, which complete the pyramids in a Ψ-octahedral arrangement, point towards the channel centres. The channels have a free diameter of about 2.5 Å and confine well-ordered pairs of water molecules which can be degassed by heating. A crystal-chemical and morphological comparison is drawn with the mineral dioptase, CU6[Si6O18]-6H2O. Lead cyclo-germanate is the first compound observed with pure [Ge6O18]12- single rings


lead cyclo-germanategermanate single Sechser ringsopen [001] channelsnonbonded waterdioptasepowder refinement