LITHIUM IN SEKANINAITE FROM THE TYPE LOCALITY, DOLNI-BORY, CZECH-REPUBLIC

The (Si,Al)-ordered Be end-member of the cordierite group, sekaninaite , from its type locality at Dolni Bory, Czech Republic, was analyzed, and found to contain 0.04 - 0.24 Li and 0.11 - 0.26 Na apfu; (Fe + Mn) /(Mg + Fe + Mn) (at.) ranges from 0.74 to 0.97. Lithium is incorporate d by the substitution (NaLi)-Na-ch-Li-VI(Mg,Fe)(-1). This sekaninaite is the first member of the cordierite group reported to have substanti al Li but negligible Be. All samples of Na,Be-, Na,Li,Be- and Na,Li-be aring cordierite-sekaninaite examined to date show a minor deficit in the tetrahedral site (0.237 in the literature, 0.036 in our data) and a slight excess of channel cations (Na,K,Ca) over the proportion of (B e,Li) (at.). The Li-bearing sekaninaite comes from rather simple grani tic pegmatites, in which the only Li-bearing minerals are rare triphyl ite and cookeite. Thus cordierite-group minerals are crystal-chemical sinks for Li. This finding stresses the need to analyze for trace ligh t elements, even in minerals from poorly fractionated and rare-element -depleted environments, provided the crystal chemistry is favorable.