What series of index minerals is characteristic of increasing metamorphic grade?
The series of index minerals characteristic of increasing metamorphic grade, starting with the lowest temperature zone, is **chlorite, biotite, garnet, sillimanite**.
Explanation:
In the study of metamorphic rocks, geologists use index minerals as indicators of the pressure and temperature conditions under which the rocks formed. These index minerals form a specific sequence based on the increasing metamorphic grade.
**Starting with the lowest temperature zone:**
1. **Chlorite:** Chlorite is a green mineral typically found in low-grade metamorphic rocks.
2. **Biotite:** As the temperature and pressure increase, chlorite is replaced by biotite, which is a brown mica mineral.
3. **Garnet:** In medium-grade metamorphic rocks, garnet starts to form, indicating higher temperatures and pressures.
4. **Sillimanite:** Finally, in the highest grade rocks, sillimanite appears, signifying the most extreme pressure and temperature conditions.
The progression from chlorite to biotite, garnet, and sillimanite reflects the increasing metamorphic grade and the changing mineral assemblages in response to these conditions. This sequence of index minerals helps geologists interpret the history of metamorphic rocks and the environments in which they formed.