Abstract: Lithium-bearing don bassite and tosudite were found in veins in hydrothermally altered granite (Beauvoir granite) in the northern part of the Massif Central, France. The two minerals are characterized by their high Li contents and low Mg and Fe contents; their structural formulae are:
( Si 3.81 Al 0.19 ) Σ=4.00 O 10 ( Al 3.81 Li 0.52 Fe 2+ Ca 0.02 Mg 0.01 Mn 0.01 ) Σ=4.38 (OH) 8 ( Na 0.07 K 0.04 ) Σ=0.11 for donbassite and ( Si 3.50 Al 0.50 ) Σ=4.00 O 10 ( Al 2.95 Li 0.22 Fe 3+ Ti 0.01 ) Σ=3.19 (OH) 5 ( Ca 0.01 Na 0.15 K 0.18 ) Σ=0.34 for tosudite.
These chemical compositions indicate that the donbassite is an intermediate member of the donbassite-cookeite solid solution series and that the tosudite consists of interstratified Li-donbassite and beidellite. Both Li-bearing minerals show thermal behavior distinct from those previously reported for dioctahedral chlorite and tosudite.
Petrographic investigation of drill cuttings from the Echassières area indicates that the two minerals were formed in an intermediate stage of hydrothermal alteration following an early stage characterized by formation of muscovite (2M1 at > 350°C and before the latest stage characterized by deposition of kaolinite and randomly interstratified illite/smectite at < 100°C. Moreover, tosudite occurs in the upper part of the granite, whereas donbassite is restricted to the lower part, suggesting the formation of tosudite at lower temperatures.