Is the Si content of phengites controlled by metamorphic grade, host rock composition or both ? In a recent paper appearing in this journal V~LDE (1967) asserts that the silica content of phengites is independent of host rock composition and is indicative of the pressure and temperature of their formation. This writer disputes V~LD~'S hypothesis and provides new evidence showing that rock composition is an important control of Si-content in phengites and muscovites. V~LD~ reports that phengites in seven specimens of glaucophane schist (from the Ile de Groix, France) have variable mineral association and A1, Fe +2, Fe +3 and Mg content but have relatively uniform Si content. From this he concludes that ".... the Si +4 content of the phengites is independent of the associated mineral phases in the same rock. The Si content is also independent of the relative abundance of the other elements in the mica. Variability in Si content of natural phengitie micas can then be attributed to the physical conditions of formation such as pressure and temperature." In his Fig. 1 V~LD~ presents "Stability curves for various phengite micas of the series K[A12Si~AIO10(OH)2 ] --K[MgA1Si4010(OH)2 ] under conditions of P~I~O---= Pto~al"' It is not clear from this diagram, or the caption, whether for each curve the stated Si content (for example 3.2) is the maximum amount of Si stable in phengites at some given T and P along the curve, or at P and T along the curve only those phengites with the stated Si content are stable. The experimental data upon which these curves are based (V~LD~, 1965) surely do not warrant the second interpretation, which from the discussion is apparently that made by V:ELDE. FOST]~R (1956) studied compositional variations in muscovites and concluded that the Si content varies according to the relationship: R +2 Si ,~ A1 R+3