Notes on leucophanite from the Ilímaussaq alkaline complex, South Greenland

Abstract

Leucophanite from the Ilímaussaq alkaline complex was first described in 1905. For nearly 60 years it was believed to be rare and of very limited distribution in Ilímaussaq, but it has lately proved to be of relatively widespread occurrence. Detailed descriptions of the various occurrences are given. The crystals show the forms {001}, {110}, {101}, {102}, {111} and {113}; all the crystals are twinned either along (110) or (010). Optically the Ilímaussaq leucophanite is biaxial with 2V_α (measured) = 40°; α = 1.575, β = γ = 1.597. The empirical formula, calculated on the basis of Si + Al = 4, is: (Ca_1.97Mg_0.05REE_0.06)_Σ2.08(Na_2.13K_0.01)_Σ2.14Be_1.92(Si_3.88Al_0.12)_Σ4.00O_12.01(F_1.87OH_0.19)_Σ2.06. Unit cell parameters determined on the newly found material are a = 7.38–7.40, b = 7.40–7.41, c = 9.96–9.95, all ± 0.01 Å (Kangerluarsuk) and a = 7.43, b = 7.43, c = 9.90, all ± 0.01 Å (Nakkaalaaq). Thus, a full set of data for the Ilímaussaq leucophanite is now available.