Here it is
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double chain silicates -There are two types of tetrahedra: those sharing 3 vertices and those sharing only 2 vertices.
So one ring has two Si not shared and 4 Silicon atoms are shared with two rings
Number of Si per unit is 2+2 =4
Number of unshared oxygen atoms are 11
The general formula of double chain silicates (or Amphiboles) is (Si₄O₁₁)_n^6n-