This specimen is an example of a lithium replacement mineral that formed in a complex pegmatite environment.
Complex Pegmatites: Lithium Replacement Minerals
During crystallization of a magma, the residual pegmatite fluids sometimes contain exceptionally large amounts of highly soluble elements. These fluids are chemically very active and can react with minerals formed during earlier stages of crystallization. The sample of beryl seen here (#12) has begun to dissolve in this way, for example.
Through their chemical activity, these fluids can likewise replace minerals formed earlier with others containing the more soluble elements — chiefly lithium, but also other rare elements such as tantalum, cesium, and thorium. Lithium replacement pegmatites, as they are called, involve a complex history of development; they are the only known source of such rare element-bearing minerals as amblygonite, lepidolite, and pollucite.
Minas Gerais, Brazil