Magma Behavior, Mineral Identification, and Rock Analysis
1. Composition and Types of Magma
The behavior and cooling of magma determine the final characteristics of igneous rocks. Magma is a complex mixture of molten silicate liquid, suspended crystals, and dissolved gases. It is primarily composed of eight elements: Silicon (Si), Oxygen (O), Aluminum (Al), Iron (Fe), Magnesium (Mg), Calcium (Ca), Sodium (Na), and Potassium (K). The Silica (SiO2) content is the most important factor in classification.
| Magma Type | Silica Content | Dominant Minerals | Resulting Rock |
|---|---|---|---|
| Mafic (Basaltic) | Low (~50%) | Olivine, Pyroxene, Ca-Plagioclase | Basalt (Extrusive) |
| Intermediate (Andesitic) | Medium (~60%) | Amphibole, Na-Plagioclase | Andesite (Extrusive) |
| Felsic (Rhyolitic) | High (~70%+) | Quartz, K-Feldspar, Muscovite | Rhyolite (Extrusive) |
2. Physical Properties of Magma
The physical behavior of magma dictates how volcanoes erupt and how intrusive bodies form.
- Temperature: Generally ranges from 700°C to 1300°C. Mafic magmas are the hottest, while Felsic magmas are relatively cool.
- Viscosity: A measure of a fluid’s resistance to flow.
- High Silica: Felsic magma is thick and sticky (like cold honey) because silica tetrahedra link into long chains.
- Low Silica: Mafic magma is runny (like motor oil) and flows easily over long distances.
- Density: Mafic magmas are denser (due to high Iron and Magnesium) than Felsic magmas. Typically, magma is less dense than the surrounding solid rock, causing it to rise toward the surface.
3. Bowen’s Reaction Series
Developed by N.L. Bowen, this concept explains the order in which minerals crystallize from cooling magma. It is divided into two branches:
- Discontinuous Branch: As temperature drops, one mineral reacts with the melt to form the next (Olivine → Pyroxene → Amphibole → Biotite).
- Continuous Branch: Describes the evolution of Plagioclase feldspar from Calcium-rich to Sodium-rich as cooling progresses.
At the lowest temperatures, both branches merge, and the remaining melt crystallizes into Potassium Feldspar, Muscovite, and finally Quartz.
4. Magmatic Differentiation
This process allows a single parent magma to produce a variety of igneous rocks as the chemistry of the remaining liquid changes during cooling.
Mechanisms of Differentiation
- Crystal Settling (Fractional Crystallization): Early-formed, heavy minerals (like Olivine) sink, leaving the melt enriched in silica.
- Magma Mixing: Two different magma bodies merge, creating a melt with intermediate composition.
- Assimilation: Rising magma melts and incorporates surrounding country rock, changing its chemical makeup.
5. Mineral Identification
Bowen’s Reaction Series acts as a map for mineral crystallization, explaining why certain minerals are commonly found together (like Quartz and Muscovite) and others are not (like Olivine and Quartz).
The Silicate Groups
| Mineral | Diagnostic Physical Properties |
|---|---|
| Olivine | Distinctive olive-green color; sugary texture; no cleavage. |
| Garnet | Deep red to brown; dodecahedral (12-sided) crystals; hardness 7.5. |
| Kyanite | Blue, bladed crystals; hardness varies (4.5–7). |
| Staurolite | Dark brown; often forms cross-shaped twins; dull luster. |
| Tourmaline | Black or colorful; triangular-round cross-section; vertical striations. |
Pyroxenes & Amphiboles
- Augite (Pyroxene): Dark green to black; short, stubby crystals; two cleavages at ~90°.
- Tremolite / Actinolite: Fibrous or needle-like crystals.
- Hornblende (Amphibole): Black; elongated crystals; two cleavages at 56° and 124° (diamond-shaped cross-section).
Sheet Silicates
- Talc: Extremely soft (Mohs 1); soapy/greasy feel; pearly luster.
- Muscovite: Clear/silvery; peels into flexible, transparent sheets.
- Biotite: Black or dark brown; peels into thin, flexible sheets.
The Feldspar Group
- Orthoclase: Pink or white; no striations on cleavage planes.
- Microcline: Often Amazonite green; exhibits tartan twinning.
- Plagioclase: White to grey; characterized by fine, parallel grooves (striations).
6. Quartz Varieties
Quartz (SiO2) exists as Macro-crystalline (visible) or Cryptocrystalline (microscopic) forms.
Macro-crystalline
- Rock Crystal: Clear and colorless.
- Amethyst: Purple/Violet (iron impurities).
- Rose Quartz: Translucent pink.
- Smoky Quartz: Translucent grey to black (natural radiation).
Cryptocrystalline
- Chalcedony: Waxy luster; translucent.
- Agate: Concentric color bands.
- Jasper: Opaque, red or brown (iron oxide).
- Chert & Flint: Hard, dense, conchoidal fractures.
7. Rock Identification
Identification relies on texture (grain arrangement) and mineralogy.
Igneous Rocks
- Granite: Phaneritic (coarse-grained); Orthoclase, Quartz, and Biotite.
- Pegmatite: Extremely coarse-grained (>1 cm).
- Dolerite: Dark, medium-grained; Augite and Plagioclase.
- Granodiorite: Similar to granite but higher Plagioclase content.
Sedimentary Rocks
- Shale: Fine-grained; characterized by fissility (splits into thin sheets).
- Sandstone: Gritty feel; sand-sized grains.
- Limestone: Reacts with acid (effervesces) due to CaCO3.
Metamorphic Rocks
- Hornblende Schist: Highly foliated; needle-like crystals.
- Fuschite Quartzite: Hard; green, chromium-rich mica.
- Hematite Jasper Quartzite: Dense; banded red and metallic layers.