Here's why:

* Silicate Structures: Silicates are minerals composed of silicon and oxygen. The basic building block is the silica tetrahedron (SiO4) where a silicon atom is bonded to four oxygen atoms. These tetrahedra can link together in various ways to form different silicate structures.

* Double Chain Structure: In amphiboles, the silica tetrahedra link together to form double chains, like two single chains that are linked together. These chains run parallel to each other, giving the mineral its characteristic fibrous or prismatic habit.

Other silicate structures include:

* Isolated Tetrahedra: Found in minerals like olivine and garnet.

* Single Chains: Found in pyroxenes.

* Sheets: Found in micas and clay minerals.

* Framework: Found in quartz and feldspars.