The basic structural unit of feldspar, zeolites, mica, and asbestos is:

• Silicate Frameworks: All natural silicates are constructed from a fundamental, three-dimensional tetrahedral coordination unit.
• Central Coordination: A central silicon ($$\text{Si}$$) atom is covalently bonded to four oxygen ($$\text{O}$$) atoms at the corners of a tetrahedron, giving a net formula of $$\text{SiO}_4^{4-}$$.

Conclusion:

Different classes of silicates (chain, sheet, or 3D network frameworks) arise solely from sharing varying numbers of oxygen corners between these individual tetrahedra.

Answer: Option A — $$\text{SiO}_4^{4-}$$