Ring Silicates Cs 4 Tm 2 [Si 4 O 12 ](OH) 2 and Cs 4 (Tm,Tb) 2 [Si 4 O 12 ](OH) 2 As Merohedral Twins

New ring silicates Cs 4 Tm 2 [Si 4 O 12 ](OH) 2 and Cs 4 (Tm,Tb) 2 [Si 4 O 12 ](OH) 2 have been synthesized by the hydrothermal method. A structural analogy is revealed between them and the previously studied silicates K 4 Sc 2 (OH) 2 (Si 4 O 12 ) and K 2 Sc[Si 2 O 6 ]F. Their crystal structures are identified as orthorhombic merohedral twins by the lost element of tetragonal symmetry. The structures of new ring silicates are related to those of fresnoite and tetragonal melilite.