Mineral alterations in spodumene and their influence on Li2O content and the visual appearance of samples from two pegmatites in eastern Minas Gerais, Brazil

The increase in research on Li deposits in recent years has led to the study of many pegmatite occurrences worldwide. Spodumene is the leading Li resource in hard rock, and its high Li2O content is essential for a desirable one. In this work, samples of fresh spodumene and altered crystals from two pegmatites in the Oriental Pegmatitic Province in southeastern Brazil were studied to investigate the secondary minerals formed under alteration processes and the difference in Li content between the unaltered and altered crystals. Cookeite – (LiAl4)(AlSi3O10)(OH)8, illite – K0.65Al2(Al0.65Si3.35O10)(OH)2 and Kaolinite are the most voluminous alteration phases and, except cookeite, the other phyllosilicates contain no Li, directly affecting its content in the crystals. Quartz is formed with cookeite and remains as a resistant mineral; the alteration sequences follow the cookeite→muscovite/illite→kaolinite or cookeite→kaolinite progressions. The unaltered crystals have >7 % Li2O, while the most altered pseudomorph (secondary minerals that retain the original shape of spodumene crystals) sample has 1.82 % Li2O and a high amount of H2O/OH. Changes in the visual aspect of the crystals are easily noted as the alterations increase. The intensity of the alteration processes (hydrothermal and weathering) in the pegmatites seems to be crucial to ore grade since Li ions easily leach out of the spodumene structure during alteration.