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.)
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.)
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Correct Answer: 45
Solution and Explanation
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%.
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