Tremolite has the formula Ca₂Mg₅(Si₄O₁₁)₂(OH)₂…

Step 1. Oxygen for each silicon. $11 / 4 = 2.75$.

Step 2. Shared corners. $2.75 = 4 - s/2$ gives $s = 2.5$. Half of the tetrahedra share 2 corners and half share 3, as in a double chain.

Step 3. Charge of a unit. The charge for each silicon is $4 - s = 1.5$ and there are 4 silicons: $6\text{-}$.

Step 4. The metal ions. $2 \times 2 + 5 \times 2 = 14$.

Step 5. Balance. Two units of Si$_4$O$_{11}$ give $-12$, and two hydroxide ions give $-2$. The sum is $14 - 12 - 2 = 0$.