Crystal structure of sapphirine-l Tc

Both polymorphs of sapphirine, namely sapphirine-l Tc and sapphirine-2M, were found in granulites near Wilson Lake (Labrador). Both minerals can be described as consisting of equivalent layers with monoclinic symmetry, translation periods a :=: 9.78, c :=: 9.93A, with f3 :=: 110.2°, the width of a layer being b., :=: 7.20 A. The layers are built up by octahedral walls connected by tetrahedral chains. Adjacent layers are related by translation vectors t 1 :=: al2 + b., c14, or t2 :=: al2 + b., + c14. Sapphirine-l Tc corresponds to the sequence t 1t 1t 1 ... (or t 2 t2 t 2 ... ), whereas sapphirine2M corresponds to the sequence t 1t2 t 1t2 ... The structures of the two polymorphs from Wilson Lake were refined with the aim to determine the distribution of magnesium, iron, and aluminium cations in octahedral sites and of aluminium and silicon cations in tetrahedral sites. The structural data indicate for both polymorphs a substantially similar distribution, which corresponds to that obtained by P. B. Moore in his study of the sapphirine from Fiskenaesset.