Igneous rocks, formed from the cooling and solidification of magma, are the building blocks of Earth’s crust. Their composition depends largely on the mineral content of magma, which is determined by the chemical makeup of silicate minerals. These minerals are divided into two broad categories: felsic (light silicates) and mafic (dark silicates).
Among these, minerals like quartz, feldspars, micas, olivine, pyroxene, and amphibole dominate igneous rock structures. Each mineral group has unique chemical properties, crystal structures, and roles in shaping rock texture and characteristics.
This article provides an in-depth study of the mineralogical composition of magma, classification of igneous minerals, and a special focus on olivine, a key mineral in basaltic and ultramafic rocks.
Common Minerals in Igneous Rocks
Igneous rocks are largely composed of silicate minerals, which are combinations of silicon dioxide (SiO₂) with aluminum, iron, magnesium, calcium, sodium, and potassium.
Felsic Minerals (Light Silicates)
Felsic minerals are high in silica (SiO₂) and rich in non-ferromagnesian elements like potassium, sodium, and aluminum.
- Quartz (SiO₂): A highly stable and abundant mineral.
- Orthoclase Feldspar (KAlSi₃O₈): A potassium-rich feldspar.
- Plagioclase Feldspar (KAlSi₃O₈; CaAl₂Si₂O₈): Contains both sodium and calcium.
- Muscovite (KAl₃Si₃O₁₀(OH)₂): A silicate mineral belonging to the mica group.
Key Features:
- High silica content.
- Light-colored (white, pink, or grey).
- Lower density compared to mafic minerals.
- Common in granite and rhyolite.
Mafic Minerals (Dark Silicates)
Mafic minerals are rich in iron (Fe) and magnesium (Mg) but lower in silica.
- Olivine ((Mg,Fe)₂SiO₄): Greenish crystal, typical in basalt and ultramafic rocks.
- Pyroxene (Ca, Fe, Mg, Al)SiO₃: Important dark silicate.
- Amphibole ((Mg,Fe,Ca,Na)Si₈O₂₂(OH)₂): Complex group of dark silicates.
- Biotite (K(Mg,Fe,Ca,Al)Si₃O₁₀(OH)₂): A dark mica mineral.
Key Features:
- Low silica content.
- Dark-colored (black or green).
- Higher density compared to felsic minerals.
- Common in basalt and gabbro.
Mineralogical Composition of Magma
As magma cools, silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) form silicate minerals.
- Mafic minerals (dark silicates): High in Fe and Mg, low in SiO₂.
- Felsic minerals (light silicates): Rich in SiO₂, low in Fe and Mg.
Key Points
- Feldspars account for 40% or more of most igneous rocks.
- Felsic minerals = highest silica content.
- Mafic minerals = lowest silica content.
Types of Magma and Rock Formation
| Magma Type | Rock Types | Chemical Composition | Temperature (°C) | Viscosity | Volatile Content |
|---|---|---|---|---|---|
| Felsic | Rhyolitic | SiO₂ 65–75%, low Fe, Mg, Ca, high K, Na | 650–800°C | High | High |
| Intermediate | Andesitic | SiO₂ 55–65%, moderate Fe, Mg, Ca, Na, K | 800–1000°C | Intermediate | Intermediate |
| Mafic | Basaltic | SiO₂ 45–55%, high Fe, Mg, Ca, low K, Na | 1000–1200°C | Low | Low |
| Ultramafic | Picritic | SiO₂ 38–45%, Fe-Mg > 8%, high MgO, low K, Na | Up to 1500°C | Very Low | Low |
Focus on Olivine
Chemical Composition and Properties
- Formula: (Mg,Fe)₂SiO₄
- Mineral Group: Orthosilicate
- Hardness: 6.5 – 7 on Mohs scale
- Specific Gravity: 3.27 – 3.37
- Color: Olive green, grayish green, or brown.
Occurrence
- Found in mafic and ultramafic rocks, especially basalt and gabbro.
- Characteristic of basaltic lava, formed from rapid cooling.
Significance
- Major rock-forming mineral in Earth’s mantle.
- High Mg and Fe content makes it an indicator of mantle-derived rocks.
- Used in refractory materials due to its high melting point.
Importance of Felsic vs Mafic Minerals
Felsic Rocks (Granite, Rhyolite):
- Light-colored, silica-rich.
- Lower density, continental crust composition.
Mafic Rocks (Basalt, Gabbro):
- Dark-colored, silica-poor.
- Higher density, oceanic crust composition.
This contrast explains why continental crust is lighter and thicker, while oceanic crust is denser and thinner.
Comparative Table – Felsic vs Mafic Minerals
| Property | Felsic Minerals | Mafic Minerals |
|---|---|---|
| Silica Content | High | Low |
| Color | Light (white, pink, grey) | Dark (green, black) |
| Density | Low | High |
| Examples | Quartz, Muscovite, Feldspar | Olivine, Pyroxene, Amphibole, Biotite |
| Rock Types | Granite, Rhyolite | Basalt, Gabbro |
FAQs
Q1. What are felsic minerals?
Felsic minerals are light-colored silicates rich in silica, potassium, and sodium, such as quartz and feldspar.
Q2. What are mafic minerals?
Mafic minerals are dark-colored silicates rich in iron and magnesium, such as olivine and pyroxene.
Q3. Why is olivine important in geology?
Olivine is a key mineral in basalt and ultramafic rocks, and its presence indicates mantle-derived magma.
Q4. Which rocks are rich in felsic minerals?
Granite and rhyolite are felsic-rich rocks.
Q5. Which rocks are rich in mafic minerals?
Basalt and gabbro are mafic-rich rocks.
