Hornblende and other amphiboles have double chain silicate structures.
Tetrahedron (SiO4)^4- is the fundamental building block of all the silicate minerals. Silicate minerals comprising chains are known as inosilicates. In single chains, the ratio of silicon to oxygen is 1: 3, while it is 4: 11 in the case of double chains.
The double-chain silicates take place when tetrahedral produce a double chain by sharing two or three oxygen atoms each.