Mineralogy & Petrology

Igneous Petrology & Bowen's Series

Silica classification, Bowen's reaction series and fractional crystallisation — how magmas evolve from basalt to granite.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Igneous rocks are classified by silica content (ultramafic <45 %, mafic 45–52 %, intermediate 52–65 %, felsic >65 %) and grain size (coarse plutonic vs fine volcanic). Bowen's reaction series describes the order of crystallisation: a discontinuous ferromagnesian branch (olivine → pyroxene → amphibole → biotite) and a continuous plagioclase branch (Ca-rich → Na-rich), converging on K-feldspar → muscovite → quartz. Fractional crystallisation removes early crystals, driving the residual melt toward silica-rich (felsic) compositions.

The Core Formula Matrix

SiO₂ classes: ultramafic <45 % · mafic 45–52 % · intermediate 52–65 % · felsic >65 %.

Plutonic ↔ volcanic pairs: granite↔rhyolite · diorite↔andesite · gabbro↔basalt · peridotite↔komatiite.

Forsterite number:

F o # = 100 \frac{M g}{M g + F e}

.

Batch melting:

C_{L} = \frac{C _{0}}{D + F ( 1 - D )}

The ‘IIT Traps’

⚠
Felsic = light, mafic = dark. Felsic rocks are silica-rich and pale; mafic are Fe–Mg-rich and dark.
⚠
Grain size ≠ composition. Granite and rhyolite share composition but differ in cooling rate.
⚠
Bowen order. Olivine crystallises first (highest T); quartz last (lowest T).

📚 Standard references

Igneous and Metamorphic Petrology — Myron G. Best
Igneous Petrology — Anthony Hall

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The fine-grained volcanic equivalent of granite is

Q2MEDIUM1 Mark · MCQ

In Bowen's discontinuous series the first ferromagnesian mineral to crystallise is

Q3HARD2 Marks · NAT

Batch melting:

C_{0} = 20

ppm, bulk

D = 0.05

, melt fraction

F = 0.1

. The melt concentration

C_{L}

is _____ ppm.

ppm