Plot of the a parameter against x for a range of Mg–Al (circles) and Mg–Ga (triangles) layered double hydroxides (LDH) reported in the literature; the data for the Mg–Al LDH involve a variety of interlayer anions (i.e. OH−, CO32−, NO3−, Cl−) and are from: Mg–Al: Bellotto et al., 1996; Bîrjega et al., 2005; Brindley & Kikkawa, 1979; Budhysutanto et al., 2011; Gastuche et al., 1967; Han et al., 1998; Jinesh et al., 2010; Kaneyoshi & Jones, 1999; Kukkadapu et al., 1997; Mascolo & Marino, 1980; Miyata, 1980; Pausch et al., 1986; Rao et al., 1998; Sato et al., 1988; Shen et al., 1994; Valente et al., 2011; Xu & Zeng, 2001; Yun & Pinnavaia, 1995; Mg–Ga: Bellotto et al., 1996; López-Salinas et al., 1997. The full lines are the result of the linear regression analyses of both sets of data and the filled diamond represents the β polymorph of magnesium hydroxide (i.e. brucite). The dashed lines represent the values of a calculated from theory [using equation (15) in Richardson, 2013b]. The open diamond can be taken to represent a theoretical α form of magnesium hydroxide (Richardson, 2013b). The bold numbers correspond to the ordered distributions of M3+ ions in the octahedral layer that are illustrated in Fig. 1; the three compositions that are labelled `x' are explained in the text.