Eudialyte group

Eudialyte group is a group of complex trigonal zircono- and, more rarely, titanosilicate minerals with general formula [N(1)N(2)N(3)N(4)N(5)]₃[M(1a)M(1b)]₃M(2)₃M(4)Z₃[Si₂₄O₇₂]O'₄X₂, where N(1) and N(2) and N(3) and N(5) = Na⁺ and more rarely H₃O⁺ or H₂O, N(4) = Na⁺, Sr²⁺, Mn²⁺ and more rarely H₃O⁺ or H₂O or K⁺ or Ca²⁺ or REE³⁺ (rare earth elements), M(1) and M(1b) = Ca²⁺, M(1a) = Ca²⁺ or Mn²⁺ or Fe²⁺, M(2) = Fe (both II and III), Mn and rarely Na⁺, K⁺ or Zr⁴⁺, M(3) = Si, Nb and rarely W, Ti and [] (vacancy), M(4) = Si and or rarely [], Z Zr⁴⁺ and or rarely Ti⁴⁺, and X = OH⁻, Cl⁻ and more rarely CO₃²⁻ or F⁻. Some of the eudialyte-like structures can even be more complex, however, in general, its typical feature is the presence of [Si₃O₉]⁶⁻ and [Si₉O₂₇]¹⁸⁻ ring silicate groups. Space group is usually R3m or R-3m but may be reduced to R3 due to cation ordering. Like other zirconosilicates, the eudialyte group minerals possess alkaline ion-exchange properties, as microporous materials.

The list of eudialyte-related natural phases is growing. There are many such phases, some of them very complex, coded "UM" by the International Mineralogical Association, and include:

...
Wikipedia