Granitic pegmatites.
Ref.: Atencio, D., Neumann, R., Silva, A.J.G.C., and Mascarenhas, Y.P. (1991) Phurcalite from Perus, São Paulo, Brazil, and redetermination of its crystal structure. Canadian Mineralogist (1991): 29: 95-105.
http://sbg.igc.usp.br/rgb/vol24_down/2401/2401043.pdf