LAUSR

The problem to be solved is whether Al is a necessary component of Fe-Mg chlorites. Very unusual Al-depleted and Fe-enriched trioctahedral chlorites with the empirical formulae Na 0.05 Ca 0.05 (Fe 2+ 3.01 Mg 2.01 Ti 0.14 Fe 3+ 0.04 ) Σ6.00 [(Si 3.53 Fe 3+ 0.41 Al 0.06 ) Σ4.00 O 10 ](OH) 8 · n H 2 O (Sample 1) and Na 0.05 Ca 0.01 (Fe 2+ 3.26 Mg 1.97 Fe 3+ 0.75 Mn 0.01 Ti 0.01 ) Σ6.00 [(Si 3.16 Fe 3+ 0.75 Al 0.09 ) Σ4.00 O 10 ](OH) 8 (Sample 2) have been discovered in Al-depleted dolomite carbonatites of the Kovdor complex of ultramafic, alkaline rocks and carbonatites, Kola Peninsula, Russia. The presence of substantial amounts of Ti in Sample 1 is another unusual feature of this mineral. In both samples, chlorites are intimately intergrown with cronstedtite-1 T which is an indication of a low stability of chlorite structure in the absence of aluminum in the tetrahedral sheet. The crystal structure of chlorite in Sample 1 was solved by the Rietveld method. The mineral is triclinic (IIb-4-module), space group C - 1 $$ 1 $$ , a = 5.4153(4), b = 9.3805(7), c = 14.5743(12) Å, α = 90.137(5)°, β = 96.928(5)°, γ = 90.043(6)°, V = 734.95(10) Å 3 , and Z = 2. A problem to be solved is how stable are Al-free chlorites belonging to the clinochlore–chamosite solid-solution series and whether their existence in natural mineral assemblages is possible. The results obtained indicate that even though Al-depleted chlorites belonging to the clinochlore–chamosite solid-solution series exist in Nature as metastable phases, these minerals are extremely rare and much less stable than Al-poor serpentines.