Question

The structural formula of phyllosilicates is

• A. $SiO_4^{4-}$
• B. $Si_2O_7^{6-}$
• C. $Si_2O_5^{2-}$
• D. $SiO_3^{2-}$

Hint:

Sheet = Phyllo.
Think of "Phyllo dough" (paper-thin sheets).
Phyllosilicates = 3 oxygen atoms shared per tetrahedron $\rightarrow$ $Si_2O_5$.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
Phyllosilicates are "sheet silicates" that form the bulk of clay minerals. The question asks for the fundamental repeating chemical unit that defines this sheet structure.
Detailed Explanation:

• Silicate Classification:
Silicates are classified based on how the $SiO_4$ tetrahedra are linked together by sharing oxygen atoms.

• Sheet Structure (Phyllosilicates):
In phyllosilicates (from Greek phyllon, meaning "leaf"), each silicon tetrahedron shares three of its four oxygen atoms with neighboring tetrahedra.
This creates an infinite two-dimensional sheet.

• Deriving the Ratio:
For each $Si$ atom, three oxygens are shared (counting as $1/2$ each) and one oxygen is unshared (counting as $1$).
Total Oxygen per $Si = 1 + (3 \times 0.5) = 2.5$.
To get a whole number formula, we double it: $Si_2O_5$.
Since $Si$ has a $+4$ charge and $5$ oxygens have $-10$, the resulting net charge for the unit is $2 \times (+4) + 5 \times (-2) = -2$.
Hence, the formula is $Si_2O_5^{2-}$.

• Other Options:
$SiO_4^{4-$:} Nesosilicates (Individual tetrahedra like Olivine).
$Si_2O_7^{6-$:} Sorosilicates (Paired tetrahedra).
$SiO_3^{2-$:} Inosilicates (Single chain like Pyroxene).

Step 2: Final Answer:
The repeating structural unit of phyllosilicates is $Si_2O_5^{2-}$.