As magma cools, minerals crystallize in a fixed order set by their melting points — Bowen's Reaction Series. Two branches run in parallel: the continuous plagioclase series gradually shifts from calcium-rich to sodium-rich feldspar as it cools, while the discontinuous ferromagnesian series jumps between distinct mineral species — olivine → pyroxene → amphibole → biotite — each reacting with the remaining melt to form the next. Both branches converge on late-stage K-feldspar, muscovite and quartz.
Tcrystal(mineral) fixed by composition
cooling rate → time at each Tcrystal
slow cooling → large crystals → plutonic (granite-like)
fast cooling → small crystals → volcanic (basalt-like)
- Cooling rate — how fast the magma loses heat. Slow cooling (deep intrusion) gives crystals time to grow large; fast cooling (surface eruption) freezes many tiny crystals almost at once.
- Continuous branch (plagioclase) — one mineral family whose composition drifts smoothly as temperature drops.
- Discontinuous branch (ferromagnesian) — a sequence of chemically distinct minerals, each forming by reaction with the cooling liquid.
- Resulting texture — the grain size you see at the end records the cooling history: coarse-grained rock crystallized slowly underground, fine-grained rock chilled quickly at or near the surface.
This series is the backbone of igneous petrology: it explains why granite, diorite, gabbro, rhyolite and basalt differ in both mineralogy and texture despite all forming from cooling silicate melts.