Original paper

Crystal structures of sapphirine and surinamite analogues in the MgO-Ga2O3-GeO2 system

Barbier, Jacques

European Journal of Mineralogy Volume 10 Number 6 (1998), p. 1283 - 1294

17 references

published: Dec 1, 1998
manuscript accepted: Jun 3, 1998
manuscript received: Nov 3, 1997

DOI: 10.1127/ejm/10/6/1283

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ArtNo. ESP147051006014, Price: 29.00 €

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Abstract

Abstract Single crystals of the sapphirine and surinamite analogues in the MgO-Ga2O3-GeO2 system have been grown from potassium molybdate fluxes. Their crystal structures and cation distributions have been determined by X-ray diffraction. Sapphirine, Mg8.9Ga14.2Ge4.9O40, crystallizes in the P1̅ space group with a = 8.822(2), b = 9.794(2), c = 10.256(2) Å, α= 63.82(3), ß = 84.77(3), γ= 65.32(3)°, Z = 1, wR(F2) = 0.080 for all 6770 measured reflections, R(F) = 0.037 for 3932 reflections with F0 > 4α(F0). The sapphirine analogue is isostructural with the triclinic mineral sapphirine- 1A, e.g. Mg6.3Fe2.1Al16.1 ,Si3.5O40. Surinamite, Mg4.05Ga4.9Ge3.05O16, crystallizes in the C2/c space group with a = 10.320(4), b = 23.733(9), c= 10.073(4) Å, ß= 110.29(1)°, Z= 8, wR(F2) = 0.086 for 4401 measured reflections, R(F) - 0.037 for 3301 reflections with F0 > 4α(F0). The C2/c structure of the surinamite analogue represents a new polytypic modification of the P2/n natural surinamite, Mg3Al4Si3BeO16. 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

Keywords

sapphirinesurinamitegermanatecrystal structurecation orderingMgO-Ga2O3-GeO2 systempolytypism