Crystal structures of sapphirine and surinamite analogues in the MgO-Ga- (sub 2) O (sub 3) -GeO (sub 2) system

Single crystals of the sapphirine and surinamite analogues in the MgO - Ga2O (sub 3) - GeO (sub 2) system have been grown from potassium molybdate fluxes. Their crystal structures and cation distributions have been determined by X-ray diffraction. Sapphirine, Mg (sub 8.9) Ga (sub 14.2) Ge (sub 4.9) O (sub 40) , crystallizes in the P1 space group with a = 8.822(2), b = 9.794(2), c = 10.256(2) Aa, alpha = 63.82(3), beta = 84.77(3), gamma = 65.32(3) degrees , Z = 1, wR(F (super 2) ) = 0.080 for all 6770 measured reflections, R(F) = 0.037 for 3932 reflections with F (sub 0) > 4sigma (F (sub 0) ). The sapphirine analogue is isostructural with the triclinic mineral sapphirine-1A, e.g. Mg (sub 6.3) Fe (sub 2.1) Al (sub 16.1) Si (sub 3.5) O (sub 40) . Surinamite, Mg (sub 4.05) Ga (sub 4.9) Ge (sub 3.05) O (sub 16) , crystallizes in the C2/c space group with a = 10.320(4), b = 23.733(9), c = 10.073(4) Aa, beta = 110.29(1) degrees , Z = 8, wR(F (super 2) ) = 0.086 for 4401 measured reflections, R(F) = 0.037 for 3301 reflections with F (sub 0) > 4sigma (F (sub 0) ). The C2/c structure of the surinamite analogue represents a new polytypic modification of the P2/n natural surinamite, Mg (sub 3) Al (sub 4) Si (sub 3) BeO (sub 16) . The crystal chemistry of the sapphirine and surinamite germanate analogues is discussed in terms of their cation distributions and their comparison with the distributions in the silicate minerals. Tetrahedral ordering is found to be more pronounced in surinamite than in sapphirine due to the particular topology of the tetrahedral chains.