CYCLOSILICATES
Cyclosilicates Ring silicates: Si:O ratio of 1:3. Beryl, Tourmaline, are primary examples; corderite has rings of silica tetrahedra bridged by aluminum tetrahedra
Beryl Be3Al2Si6O18 Hexagonal
Tourmaline (Na,Ca)(Li,Mg,Al)(Al,Fe,Mn) 6(BO3)3(Si6O18)(OH)4 Hexagonal
Cordierite (Mg,Fe)2Al4Si5O18¥nH2O Orthorhombic
*Cordierite may also be considered as a tectosilicate; cordierite is pseudohexagonal,
and the high-temperature polymorph is truely hexagonal.
SOROSILICATES
Sorosilicates Si:O ratio of 2:7. The epidote group minerals; ÒbowtieÓ arrangements of silica tetrahedra
Zoisite Ca2Al3O(SiO4)(Si2O7)(OH) Orthorhombic
Clinozoisite Ca2Al3O(SiO4)(Si2O7)(OH) Monoclinic
Epidote Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH) Monoclinic
Allanite (Ca,Ce) 2(Fe,Fe)Al2O(SiO4)(Si2O7)(OH) Monoclinic