“…For sodium, Na 2 MTi(PO 4 ) 3 (M = Fe, Cr) were obtained and characterized by Isasi & Daidouh (2000), while the group of potassium-containing langbeinites is the largest: KTi 2 (PO 4 ) 3 (Masse et al, 1972), K 2 Ti 2 (PO 4 ) 3 (Leclaire et al, 1989), K 1 + x Ti 2 À y Al y (PO 4 ) 3 (Slobodyanik et al, 1991), K 2 CrTi(PO 4 ) 3 (Boudjada & Perret, 1977;Norberg, 2002), A 2 MTi(PO 4 ) 3 , A = K, Rb, Tl; M = Cr, Fe (Perret & Boudjada, 1979), K 2 VTi (PO 4 ) 3 (Rangan & Gopalakrishnan, 1994), K 2 MTi(PO 4 ) 3 , M = Er, Yb, Y (Norberg, 2002), K 2 M 0.5 Ti 1.5 (PO 4 ) 3 , M = Mn, Co, Ni Ogorodnyk et al, 2007), K 2 MZr(PO 4 ) 3 , M = Y, Gd (Wulff et al, 1992), K 2 FeZr(PO 4 ) 3 (Orlova et al, 2003), K 2 LnZr(PO 4 ) 3 , Ln = Ce-Lu (Trubach et al, 2004b), K 2 BiHf(PO 4 ) 3 (Losilla et al, 1998) (Lajmi et al, 2003). Rubidium-containing langbeinites are much more scarce: Rb 2 FeZr(PO 4 ) 3 (Trubach et al, 2004a), Rb 2 YbTi(PO 4 ) 3 and Rb 2 Yb 0.32 Ti 1.68 (PO 4 ) 3 (Gustafsson et al, 2005), Rb 2 Mg 0.5 Zr 1.5 (PO 4 ) 3 (Orlova et al, 2005), Rb 2 YTi(PO 4 ) 3 (Gustafsson et al, 2006). Cs 2 Mg 0.5 Zr 1.5 (PO 4 ) 3 (Orlova et al, 2005), however, is the only example with caesium cations in the cavities.…”