Step 1: Understanding hematite.
Hematite is an iron oxide mineral with the chemical formula Fe₂O₃. The iron in hematite exists in the +3 oxidation state (Fe³⁺).
Step 2: Conclusion.
The valency of iron in hematite is 3.
In the garnet formula \( \text{(Fe}_{2.5}\text{Mg}_{0.3}\text{Ca}_x\text{)}\text{Al}_2\text{Si}_3\text{O}_{12} \), x represents the number of atoms of Ca. The mole % of grossular in the garnet is ........... (answer in one decimal place).
Based on 8 oxygen atoms, the number of silicon atoms in a plagioclase of composition \( \text{Ab}_{20}\text{An}_{80} \) is .......... (answer in one decimal place).