BAVENITE, CA4BE3AL [SI9O25(OH)(3)], FROM THE ILIMAUSSAQ ALKALINE COMPLEX, SOUTH GREENLAND

The relatively rare mineral bavenite has been identified in the Ilimau ssaq alkaline complex; it is the first finding of bavenite in Greenlan d. The Ilimaussaq bavenite is from a well-defined, marginal zone of pn eumatolytic alteration on the north coast of the Kangerdluarssuk (Kang erluarsuk) Fjord. It is associated with albite, ilvaite, aegirine, fer rian epidote, andradite, calcite and other minerals. The Ilimaussaq ba venite forms aggregates up to nearly 2 cm large. The crystals show the forms {100}, {001}, {310}, {201} and {111}; they are platy parallel t o (100) and moderately elongated after [001]. They are optic biaxial p ositive with 2V(gamma) = 18 +/- 2 degrees, n(alpha) = n(beta) = 1.5845 , n(gamma) = 1.5957 all +/- 0.0002. The optical orientation is alpha = b, beta = a and gamma = c. Inclined, irregular extinction and extinct ion dispersion on (100) and (001) suggest lower symmetry than the orth orhombic found by X-rays. The unit cell dimensions are: a = 23.212 +/- 0.003, b = 4.993 +/- 0.001 and c = 19.480 +/- 0.003 Angstrom. Both op tical and X-ray data agree with bavenite from other localities. The ch emical composition was determined by electron microprobe, optical emis sion analysis for Be, and CHN analysis: SiO2 = 58.55(.30), Al2O3 = 5.6 0(.08), CaO = 24.25(.24), BeO = 9.5(.8), H2O = 1.8(.1), Total = 99.7 c orresponding to the empirical formula Ca4Be3.5Al1.0[Si9O26.1(OH)(1.9)] . The bavenite from Kangerdluarssuk is thus the most Be-rich and Al-po or reported up to now.