Index Minerals & Metamorphic Grade

Reading the peak temperature of a rock from the minerals it grew.

Intro Uni Geology
⏱️ About 18 min
Index Minerals & Metamorphic Grade — illustration
Illustrative image (AI-generated).

Walking across the Scottish Highlands, a geologist crosses from soft slate into glittering schist and finally into banded gneiss. Each zone contains a characteristic mineral — chlorite, then biotite, then garnet — like a thermometer frozen in stone.

💡
The big idea: Metamorphic grade reflects the peak temperature a rock reached. Index minerals appear and are consumed in a predictable sequence with rising temperature. A single rock records only its peak conditions; the full Barrovian zone sequence is a regional field gradient across many rocks.
🎯 By the end, you'll be able to
  • Define metamorphic grade and explain temperature as the dominant control
  • List the classic Barrovian index minerals in order of increasing grade
  • Use index minerals to estimate the peak metamorphic temperature of a rock
  • Explain why one rock does not pass through every Barrovian zone
  • Relate metamorphic facies to combinations of pressure and temperature
📎 Helpful to know first

What is metamorphic grade?

Metamorphic grade is a measure of the intensity of metamorphism experienced by a rock. It is expressed informally as low, medium, or high grade, and it corresponds primarily to the peak temperature the rock reached during its metamorphic history.

Low-grade rocks such as slate still look a lot like their shale protoliths. High-grade rocks such as gneiss have been so thoroughly recrystallised that the original sedimentary or igneous texture is completely erased. Grade is not the same as facies — facies are formally defined P-T fields, whereas grade is a qualitative temperature scale.

🔑 Grade is mainly about temperature
While both pressure and temperature affect metamorphism, grade is primarily a temperature concept. A rock buried to 40 km in a cold subduction zone experiences high pressure but relatively low temperature — it is high-pressure, low-grade. A rock at 20 km in a hot orogen may be medium-pressure, high-grade.

Index minerals: nature's thermometer

Certain minerals appear only within specific temperature ranges in rocks of a given composition. These are called index minerals, and they allow geologists to read the peak temperature of a rock as easily as reading a thermometer.

In the classic Barrovian sequence — defined in the Scottish Highlands from the study of pelitic (clay-rich) rocks — the index minerals appear in this order as temperature rises:

  1. Chlorite — lowest grade, ~300–400 °C
  2. Biotite — low to medium grade, ~400–500 °C
  3. Garnet — medium grade, ~500–600 °C
  4. Staurolite — medium-high grade, ~550–650 °C
  5. Kyanite — high grade, ~600–700 °C
  6. Sillimanite — highest grade, >650 °C

Each mineral marks a isograd — a line on a map connecting points of equal metamorphic grade. Crossing an isograd means entering a new metamorphic zone.

The Barrovian sequence

The Barrovian zones were first mapped by George Barrow in the Scottish Highlands in the late 19th century. He noticed that as he walked from the edge of the metamorphic belt toward its hot core, the rocks changed in a predictable way:

  • Chlorite zone: slate with chlorite and muscovite.
  • Biotite zone: phyllite and fine schist with visible biotite.
  • Garnet zone: schist with conspicuous red garnet porphyroblasts.
  • Staurolite zone: schist with staurolite and kyanite beginning to appear.
  • Kyanite zone: coarse schist and gneiss with abundant kyanite.
  • Sillimanite zone: gneiss and migmatite with sillimanite needles.

This sequence is the textbook example of progressive regional metamorphism in a continent-continent collision setting.

⚠️ More pressure does not always mean higher grade
It is tempting to think that deeper burial (more pressure) automatically raises grade. In reality, grade tracks temperature. A rock in a cold subduction zone at 100 km depth experiences enormous pressure but relatively low temperature, so it remains low-grade (blueschist facies). Conversely, a rock at 20 km in a hot mountain belt can reach high grade (amphibolite or granulite facies).
✨ One rock, one peak assemblage
A single protolith does not sequentially pass through every Barrovian zone. As temperature rises, earlier index minerals are consumed by chemical reactions to make later ones. For example, chlorite is used up to produce biotite; biotite and chlorite react to form garnet. A rock that reached garnet grade does not still contain chlorite from its earlier history. The full chlorite → biotite → garnet → staurolite → kyanite → sillimanite sequence is a regional field gradient across many rocks at different peak conditions, not a timeline inside one rock.
🎮 Metamorphic-Grade Path Simulator LIVE

Interactive P–T path simulator showing stable index minerals along a metamorphic trajectory, with a regional cross-section view of Barrovian zones.

Drag a single pelitic protolith along a P–T path and watch index minerals appear and disappear. Switch to the regional view to see Barrovian zones as a spatial field gradient across many rocks.

Metamorphic facies: the P–T framework

While grade is a qualitative temperature scale, metamorphic facies are formally defined regions on a pressure-temperature diagram. Each facies is named after a characteristic mineral assemblage in a specific protolith:

  • Greenschist facies: low to medium grade; chlorite, epidote, and actinolite in mafic rocks.
  • Amphibolite facies: medium to high grade; hornblende and plagioclase in mafic rocks.
  • Granulite facies: very high grade; pyroxene and garnet, indicating dry, hot conditions.
  • Blueschist facies: high pressure, low temperature; glaucophane — signature of subduction zones.
  • Eclogite facies: very high pressure; omphacite and pyrope garnet — deep subduction or collision.

Facies link protolith, peak P-T conditions, and mineral assemblage into one coherent framework.

Barrovian metamorphic zones arranged spatially from low-grade chlorite to high-grade sillimanite toward the orogen core Chlorite zone Biotite zone Garnet zone Staurolite zone Kyanite zone Sillimanite zone ~300 °C ~400 °C ~500 °C ~575 °C ~650 °C ">700 °C → Increasing temperature toward the orogen core ● ● ● ● ● ● Barrovian zones — Scottish Highlands style field gradient
The classic Barrovian zonation in the Scottish Highlands. Each isograd marks the first appearance of an index mineral, mapping increasing temperature toward the metamorphic core.
📝 Worked example: A geologist mapping a traverse in a metamorphic belt finds chlorite-bearing slate, then biotite phyllite, then garnet schist, and finally kyanite gneiss. If the chlorite zone represents ~350 °C and the kyanite zone represents ~650 °C, what is the approximate temperature increase per zone crossed?
  1. There are 3 temperature steps between the 4 zones mentioned: chlorite → biotite → garnet → kyanite.
  2. Total temperature increase = 650 − 350 = 300 °C.
  3. Average step = 300 ÷ 3 = 100 °C per zone.
  4. This is an approximation; real boundaries depend on pressure and bulk composition, but the trend is robust.
✓ Approximately 100 °C per zone (≈ 300 °C total increase over 3 steps).
✏️ Practice: A pelitic rock is metamorphosed from 300 °C (chlorite zone) to 550 °C (garnet zone) at a constant heating rate of 10 °C per million years. How many million years does it take to enter the garnet zone?
million years
Solution
  1. Temperature increase needed = 550 − 300 = 250 °C.
  2. Time = 250 °C ÷ 10 °C per million years.
  3. = 25 million years.
✏️ Practice: Calculate the lithostatic pressure at 30 km depth in crust with average density 2750 kg/m³. Use g = 9.81 m/s² and give your answer in GPa. (1 GPa = 10⁹ Pa.)
GPa
Solution
  1. P = ρgh = 2750 × 9.81 × 30000.
  2. = 8.093 × 10⁸ Pa.
  3. = 8.093 × 10⁸ ÷ 10⁹ = 0.81 GPa.

Check your understanding

1. Which index mineral indicates the highest metamorphic grade in the classic Barrovian sequence?
In the Barrovian sequence, sillimanite appears at the highest temperatures (>650 °C), after chlorite, biotite, garnet, staurolite, and kyanite.
2. Why does a single rock NOT contain every Barrovian index mineral from chlorite to sillimanite?
As temperature rises, earlier minerals such as chlorite are used up in reactions that produce biotite and garnet. The peak assemblage replaces earlier ones; minerals do not simply accumulate.
3. A mafic rock in a cold subduction zone contains glaucophane. Which metamorphic facies does this indicate?
Glaucophane is the signature mineral of the blueschist facies, which forms under high pressure and relatively low temperature — exactly the conditions in a subduction zone.
🔎 Related lessons
Previous: Foliation: Aligned Minerals Under Pressure Next: From Protolith to Product Earth's Layers: Crust, Mantle, Core
✅ Key takeaways
  • Metamorphic grade reflects the peak temperature a rock reached; it is primarily a temperature concept.
  • Barrovian index minerals appear in order: chlorite → biotite → garnet → staurolite → kyanite → sillimanite.
  • A single rock records only its peak assemblage; earlier minerals are consumed by reactions, not retained.
  • The full Barrovian sequence is a regional field gradient across many rocks at different peak conditions.
  • Metamorphic facies (greenschist, amphibolite, granulite, blueschist, eclogite) formally map P-T conditions to mineral assemblages.
➡️ Index minerals and facies tell us the conditions. The final step is to connect those conditions back to the original rock — because every metamorphic rock started as something else.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of metamorphic rocks.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.