Question

The following table shows modal abundance and mineral composition data of a plutonic igneous rock. The amount of SiO2 in bulk composition of the rock is ________ %. (Round off to two decimal places.)

Hint:

Bulk chemical composition = weighted average of mineral modes × compositions.

Answer 1

Correct Answer: 45

Step 1: Weighted average formula. 
\[ \text{Bulk SiO}_2 = \frac{\sum (\text{Mode} × \text{SiO}_2)}{100} \] Step 2: Substitution. 
\[ (45 × 34) + (35 × 55) + (20 × 58) = 1530 + 1925 + 1160 = 4615 \] \[ \text{Bulk SiO}_2 = 4615 / 100 = 46.15%. \] Step 3: Conclusion. 
Bulk SiO2 = 46.15%.