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

Strategy: Identify the basic structural unit (monomeric unit) common to all silicate minerals: feldspar, zeolites, mica, and asbestos.

Step 1: Silicate Structure All silicate minerals are built from the $[\ce{SiO4}]^{4-}$ tetrahedron as the fundamental unit. In this unit:

• Silicon is at the centre in tetrahedral geometry ($sp^3$)
• Four oxygen atoms surround it
• Overall charge: $4(\text{Si}) + 4(-2)(\text{O}) = 4 - 8 = -4$, so the unit is $[\ce{SiO4}]^{4-}$

Step 2: How Minerals Differ Different silicate minerals arise from sharing of corners of these tetrahedra:

• Feldspar, mica, asbestos, zeolites all derive their structures from corner-sharing of $[\ce{SiO4}]^{4-}$ units in different ways.

The basic building block common to all = $\ce{[SiO4]^{4-}}$

$\boxed{\text{Answer: (A) }[\ce{SiO4}]^{4-}}$