The basic structural unit of feldspar, zeolites, mica and asbestos is
(a) (SiO3)2-
(b) SiO2
(c) (SiO

Q: Why isn't $SiO_2$ the basic unit? Isn't sand just silica?

$SiO_2$ is the empirical formula for quartz (silica), which is a 3D network where all 4 oxygens of every tetrahedron are shared. However, the discrete, fundamental building block that repeats to form that network is still the $(SiO_4)^{4-}$ tetrahedron.

Q: Where does the $4-$ charge come from in $(SiO_4)^{4-}$?

Silicon is in a $+4$ oxidation state, and each of the four oxygen atoms is in a $-2$ state. The total charge is the sum: $+4 + 4(-2) = -4$.

Q: How do these negatively charged units form stable, solid minerals?

The negative charges on the unshared (terminal) oxygen atoms are balanced by positively charged metal cations (like $Mg^{2+}$, $Fe^{2+}$, $K^+$, or $Ca^{2+}$) that sit in the spaces between the silicate structures, holding the whole crystal together via ionic bonds.

Q: Are zeolites also silicates? I thought they had aluminum.

Yes, zeolites are a specific type of framework silicate called 'aluminosilicates'. In their 3D structure, some of the $Si^{4+}$ ions in the tetrahedra are replaced by $Al^{3+}$ ions, which creates a net negative charge on the framework that is balanced by trapped cations.

Q: Why is silicon always tetrahedral in these minerals instead of forming double bonds like carbon in $CO_2$?

Silicon is much larger than carbon. Its $3p$ orbitals are too large and diffuse to form strong, stable $p\pi-p\pi$ double bonds with oxygen. Instead, it prefers to form four strong single ($sp^3$) bonds, resulting in a tetrahedral geometry.

Question

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

Options:

(A) $(SiO_3)^{2-}$

(B) $SiO_2$

(C) $(SiO_4)^{4-}$

(D) $\left( \begin{array}{c} R \ | \ -Si-O- \ | \ R \end{array} ight)_n (R=Me)$

Sigma Percentile · Accepted Answer

### The Architecture of the Earth's Crust

When you look at a piece of granite, the shimmering flakes of mica, or the fibrous strands of asbestos, you are looking at the masterpieces of inorganic chemistry. The Earth's crust is overwhelmingly composed of silicate minerals. But despite their vast diversity in appearance and physical properties, they all share a single, elegant architectural secret.

Just as a magnificent Lego castle is built from individual, identical bricks, the complex structures of feldspar, zeolites, mica, and asbestos are all constructed from one fundamental repeating unit: the **orthosilicate ion**, mathematically represented as $(SiO_4)^{4-}$.

### Inside the Silicate Tetrahedron

Let's zoom in on this foundational block. At the heart of the $(SiO_4)^{4-}$ unit lies a single silicon atom. Silicon, being a Group 14 element, utilizes $sp^3$ hybridization to form four strong covalent bonds. It surrounds itself with four oxygen atoms, pushing them as far apart as possible to minimize electron repulsion. The result is a perfect geometric **tetrahedron**.

Because silicon is in a $+4$ oxidation state and each oxygen is in a $-2$ oxidation state, the entire tetrahedral unit carries a net charge of $-4$. This highly charged unit is the starting point for all silicate chemistry.

### The Art of Sharing: Bridging vs. Terminal Oxygens

How do we get from a simple tetrahedron to the complex sheets of mica or the 3D frameworks of zeolites? The answer lies in sharing.

Silicate tetrahedra can link together by sharing their corner oxygen atoms.

*   **Terminal Oxygens:** If an oxygen atom is bonded to only one silicon atom, it is called a terminal oxygen. It retains its negative charge, which is usually balanced by metal cations like $Na^+$, $K^+$, or $Ca^{2+}$ in the crystal lattice.
*   **Bridging Oxygens:** If an oxygen atom is shared between two silicon atoms, forming a $Si-O-Si$ linkage, it is called a bridging oxygen.

The incredible variety of silicate minerals arises purely from *how many* of the four oxygen atoms are shared:

1.  **Orthosilicates:** Zero oxygens shared. They exist as discrete $(SiO_4)^{4-}$ units.
2.  **Pyrosilicates:** One oxygen shared, forming $(Si_2O_7)^{6-}$ islands.
3.  **Chain Silicates (like Asbestos):** Two oxygens shared, forming long, fibrous chains.
4.  **Sheet Silicates (like Mica):** Three oxygens shared, forming flat, easily cleavable 2D sheets.
5.  **Framework Silicates (like Feldspar and Zeolites):** All four oxygens shared, creating a robust, 3D network.

### Final Conclusion

No matter how intricately the tetrahedra are woven together, the basic structural unit that defines the entire class of these minerals is the $(SiO_4)^{4-}$ tetrahedron. Therefore, understanding this single unit unlocks the chemistry of the very ground we walk on.

Animated Solution for Chemistry - s and p-Block Elements: The basic structural unit of feldspar, zeolites, mica and asbestos is

Select Answer:

Visualized Solution

$Identifying the Minerals$

$The Fundamental Building Block$

$Geometry of the Silicate Unit$

$Linking the Tetrahedra$

$Conclusion$

$The Way Forward$

The Sigma Insight: Group 14 Elements

The Architecture of the Earth's Crust

Inside the Silicate Tetrahedron

The Art of Sharing

Final Conclusion

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