Question

What is the empirical formula of sheet silicates?
 \[\begin{align}
  & (A){{(S{{i}_{2}}{{O}_{5}})}_{n}}^{2n-} \\
 & (B)N{{O}_{2}},{{O}_{2}} \\
 & (C){{(Si{{O}_{3}})}_{n}}^{n-} \\
 & (D){{(S{{i}_{2}}{{O}_{7}})}_{n}}^{3n-} \\
\end{align}\]

Answer 1

Hint: The basic building block of all silicate minerals is the ${{[Si{{O}_{4}}]}^{4-}}$ tetrahedron. Silicate minerals containing chains are termed insolicates. In single chains ${{(Si{{O}_{3}}^{2-})}_{n}}$ , the silicon to oxygen ratio 1:3. While in double chains ${{(S{{i}_{4}}{{O}_{11}}^{6-})}_{n}}$ , the silicon to oxygen ratio 4:11.

Complete answer:
Silicates are the mineral containing silicon and oxygen in tetrahedral ${{[Si{{O}_{4}}]}^{4-}}$ units, in which silicon atom is bonded to four oxygen atoms in tetrahedral fashion. In silicates either the distance, the unit is present or many such units are joined together via corners by sharing 1, 2, 3, or 4 oxygen atoms per silicate unit.
Some of the examples are feldspar, zeolites, mica, and asbestos. The important man-made silicates are glass and cement.
When silicate chains are linked together, they form chain, ring, sheet, or three-dimensional structures. The negative charge on the silicate structure is neutralized by positively charged metal ions. If all four corners are shared with other tetrahedral units, three-dimensional networks are formed.
Depending on the way the tetrahedral units are linked, the silicates are classified into the following types.
1. orthosilicates
2. pyro silicates
3. Cyclic silicates
4. Chain silicates
5. double chain silicates
6. Silicon sheets or phyllosilicates
7. Three-dimensional silicates.
Silicon sheets or phyllosilicates:
The general formula of sheet or phyllo or two-dimensional 2D silicates is ${{(S{{i}_{2}}{{O}_{5}})}_{n}}^{2n-}$. Each ${{[Si{{O}_{4}}]}^{4-}}$ tetrahedron unit shares three oxygen atoms with others and thus by forming two-dimensional sheets. These silicates can be cleaved easily just like graphite. The layers are held together by weak Vander Waal's forces.

Hence, the empirical formula of sheet silicates ${{(S{{i}_{2}}{{O}_{5}})}_{n}}^{2n-}$, option A.

Note:
 If aluminum replaces a few with silicon atoms in a three-dimensional network of silicon dioxide, the overall structure known as aluminum silicates acquires a negative charge. Cations such as $N{{a}^{+}},C{{a}^{+2}}or{{K}^{+}}$ balance the negative charge. Examples are feldspar, zeolites.