Identifying Minerals in Igneous Rocks

## Minerals in Igneous Rocks

Igneous rocks form from the cooling and crystallization of magma or lava. The mineral content depends on the magma's chemical composition (felsic vs. mafic) and cooling rate (intrusive vs. extrusive).

## Bowen's Reaction Series

Norman Bowen discovered that minerals crystallize from magma in a predictable sequence. This is the key to understanding which minerals occur together.

### Discontinuous Series (Mafic Minerals)

| Temperature | Mineral | Formula | Crystal System | Mohs |
|-------------|---------|---------|----------------|------|
| ~1200°C | Olivine | (Mg,Fe)₂SiO₄ | Orthorhombic | 6.5–7 |
| ~1100°C | Pyroxene (augite) | Ca(Mg,Fe)Si₂O₆ | Monoclinic | 5–6 |
| ~1000°C | Amphibole (hornblende) | Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂ | Monoclinic | 5–6 |
| ~800°C | Biotite | K(Mg,Fe)₃AlSi₃O₁₀(OH)₂ | Monoclinic | 2.5–3 |

### Continuous Series (Plagioclase Feldspar)

| Temperature | Composition | Name |
|-------------|------------|------|
| ~1500°C | Ca-rich | Anorthite |
| | | Bytownite |
| | | Labradorite |
| | | Andesine |
| | | Oligoclase |
| ~800°C | Na-rich | Albite |

## Rock Type → Mineral Content

### Felsic Igneous Rocks (Granite, Rhyolite)

| Mineral | % Typical | How to Identify in Rock |
|---------|----------|------------------------|
| Quartz | 20–35% | Glassy, conchoidal fracture, clear to gray |
| Orthoclase | 25–50% | Pink or cream, two cleavages at 90° |
| Plagioclase | 10–25% | White, striations on cleavage face |
| Biotite | 5–15% | Black flakes, perfect cleavage |
| Muscovite | 0–10% | Silvery flakes, transparent sheets |

### Mafic Igneous Rocks (Basalt, Gabbro)

| Mineral | % Typical | How to Identify in Rock |
|---------|----------|------------------------|
| Plagioclase | 40–60% | Lath-shaped, white to gray |
| Pyroxene | 20–40% | Short prismatic, dark green-black |
| Olivine | 0–20% | Olive green granules (in basalt) |
| Magnetite | 5–15% | Black, magnetic grains |

### Pegmatites (Coarse-Grained)

Pegmatites are the collector's paradise — extremely coarse-grained igneous rocks with crystals often exceeding 10 cm.

| Mineral | Formula | Crystal System | Pegmatite Type |
|---------|---------|----------------|----------------|
| Tourmaline | Complex borosilicate | Trigonal | LCT pegmatites |
| Beryl | Be₃Al₂Si₆O₁₈ | Hexagonal | LCT pegmatites |
| Spodumene | LiAlSi₂O₆ | Monoclinic | Li-rich pegmatites |
| Topaz | Al₂SiO₄(F,OH)₂ | Orthorhombic | F-rich pegmatites |

## Identification Flowchart for Igneous Minerals

1. **Is the rock light-colored (felsic)?** → Look for quartz, feldspar, muscovite
2. **Is the rock dark-colored (mafic)?** → Look for pyroxene, olivine, magnetite
3. **Are crystals very large (>2 cm)?** → Pegmatite — check for rare species
4. **Are crystals visible at all?** → If not, extrusive rock — identify by rock composition