Final answer:
In double-chain silicates, silica tetrahedra share two oxygen atoms, creating a stable chain-like structure with a silicon-to-oxygen ratio of 1 to 3 (SiO3).
Step-by-step explanation:
In double-chain silicates, silica tetrahedra are structured such that each tetrahedron shares two oxygens with adjacent tetrahedra. These shared oxygens create a repeating chain-like structure. To visualize it, think of each tetrahedron being connected to its neighbors at two corners, where each corner represents an oxygen atom. This allows for a stable formation, and it results in a silicon-to-oxygen ratio of 1 to 3 (SiO3), when considering a long chain of tetrahedra in a silicate mineral. In summary, for a double-chain silicate structure, silica tetrahedra share two oxygen atoms.