The universal mineral formula for natural mullites presented here contains a typing error. The mineral formula, given on p. 358 (Sect. 4.1), in the heading of Table 7a (p. 361) and at the end of Sect. 4.2 (p. 363), is stated as M${}_y^{+}$Mg${}_z^{\mathrm{2}+}$M${}_w^{\mathrm{2}+}$M${}_{\mathrm{4}+\mathrm{2}x+y+w-\mathrm{2}z}^{\mathrm{3}+}$M${}_{\mathrm{2}-\mathrm{2}x-y-\mathrm{2}w+z}^{\mathrm{4}+}$O_10−x.

However, the indexing of M³⁺ is erroneous. The parameter w must contain a factor of 2. The correct formula is M${}_y^{+}$Mg${}_z^{\mathrm{2}+}$M${}_w^{\mathrm{2}+}$M${}_{\mathrm{4}+\mathrm{2}x+y+\mathrm{2}w-\mathrm{2}z}^{\mathrm{3}+}$M${}_{\mathrm{2}-\mathrm{2}x-y-\mathrm{2}w+z}^{\mathrm{4}+}$O_10−x.

Consequently, also the formula Na_yMg_zCa_wAl${}_{\mathrm{4.5}+y+w-\mathrm{2}z}$Si${}_{\mathrm{1.5}-y-\mathrm{2}w+z}$O_9.75 given on p. 359 must be stated as Na_yMg_zCa_wAl${}_{\mathrm{4.5}+y+\mathrm{2}w-\mathrm{2}z}$Si${}_{\mathrm{1.5}-y-\mathrm{2}w+z}$O_9.75.

The citation “Angel and Prewitt (1987)” on p. 357 is incorrect. It is “Angel and Prewitt (1986)”.