5. In silicate structure replace SIO4 aluminum-oxygen tetrahedrons aluminum-silicate aluminum oxygen aluminum-oxygen So, the cn of aluminum could be 4 or 6 6. The chemic expression or mineral molecular formula: AL2O3 2SiO2 H,O, Zro2 SiO2 structural formula: AL2SiO5(OH)4, Zrlsio5. In silicate structure: So, the CN of aluminum could be 4 or 6 6. The chemic expression or mineral. molecular formula : Al2O3·2SiO2·H2O ; ZrO2·SiO2 structural formula : Al2 [Si2O5 ](OH)4 ; Zr[SiO4 ] aluminum + oxygen aluminum-oxygen tetrahedrons replace SiO4 tetrahedrons aluminum-silicate aluminum-oxygen octahedron