| Description |
Cookeite occurs in pseudomorphs after sekaninaite associated with muscovite, quartz, tourmaline, albite, nontronite, and chlorite in pegmatite No. 1 (proximal cookeite type), and in pockets lined with albite and quartz in the Oldrˇich pegmatite (distal cookeite type), Dolní Bory-Hateˇ, Bory pegmatite district, Moldanubian Zone, Czech Republic. The mineral assemblages of the Bory pegmatites with common Al,Fe-rich minerals (annite, schorl, sekaninaite, andalusite, diaspore, ilmenite, and almandine) suggest their primitive, peraluminous, LCT geochemical signature. Proximal cookeite forms aggregates of sheets, up to B500 mm in diameter, intergrown with prevailing muscovite and quartz. Beige to grayish, porous aggregates of distal cookeite, up to 15 cm in size, consist of thin sheets, up to B70 mm in diameter, locally associated with small grains of pyrite and pale brown siderite as a matrix. Electron microprobe data of cookeite from the No. 1 and Oldrˇich pegmatites yielded very similar results: Si (3.23–3.27, 3.21–3.32 apfu) and Al (4.59–4.65, 4.48–4.65 apfu), respectively, and low concentrations of Fe (Fetot p 0.08 apfu), Mg (p 0.07 apfu), and K (p 0.15 apfu). Concentrations of Li, B, and Be obtained by LA-ICP-MS (and wet chemical analysis— Li, Oldrˇich) of cookeite from both pegmatite dikes yielded an average of 2.22 wt.% Li2O (0.78 apfu) and 2.38 wt.% Li2O (0.84 apfu), low contents of B = 370–780 ppm and 250–430 ppm, and very low Be p16 ppm and p1.2 ppm for pegmatite No. 1 and the Oldrˇich pegmatite, respectively. Strong hydrothermal alteration of common Li-bearing sekaninaite (0.11–0.55 wt.% Li2O) is evident and a potential source of Li for both the proximal and distal cookeites. Cookeite precipitated from hydrothermal fluids at T B350–270 °C and P o B2 kbar. These data are consistent with the cookeite formation in granitic pegmatites and the distal cookeite demonstrates the high mobility of Li in hydrothermal conditions.
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