The layered silicate compound CaAl₂(Al₂Si₂)O₁₀(OH)₂ contains a double aluminosilicate layer, with both Si and Al in four-coordinate sites. Sketch an edge-on view of a reasonable structure for the double layer, involving only vertex sharing between the SiO₄ and AlO₄ units. Discuss the likely sites occupied by Ca²⁺ in relation to the silica–alumina double layer.