(Sep 2nd) Igneous Rocks, Mineral Classification, and Magma Generation Mechanics

Mineral Chemical Classification and Nomenclature

Mineral chemical formulas ending with sulfur ( $S$ ) belong to the sulfide class.
Mineral chemical formulas ending with sulfur followed by oxygen ( $S$ followed by $O$ ) belong to the sulfate class.
Distinction rules for chemical formulas:
- Ending in purely $S$ indicates a sulfide.
- Ending in $S$ and $O$ together indicates a sulfate.
Native element exception: A mineral composed entirely of a single element, such as pure sulfur ( $S$ ), is classified as a native element, not a sulfide.
Carbonate misconception: Minerals composed entirely of carbon (such as diamond or graphite) are native elements rather than carbonates, despite being fully made of carbon.
Rocks versus Minerals:
- The individual components of a rock are bound together from mineral grains.
- The five defining properties of minerals apply to the individual mineral constituents inside a rock body.
- Granite is a classic example of an igneous rock composed of multiple distinct mineral grains.

Primary Rock Types and Igneous Rock Fundamentals

Three primary classifications of rocks:
1. Igneous rocks
2. Metamorphic rocks
3. Sedimentary rocks
Etymology and importance of igneous rocks:
- Igneous derives from the Latin word ignis, meaning fire.
- Informally termed fire rocks due to their formation from melted material associated with volcanic processes.
- Igneous rocks serve as the primary focus for studying metal deposit formation and earth resources.
Subsurface versus Surface molten rock terminology:
- Magma: Molten rock located below the surface of the Earth.
- Lava: Molten rock that spews or erupts onto the surface of the Earth.
- Subsurface conditions maintain significantly higher temperatures than surface environments.

Igneous Rock Classification Systems

Classification System 1: Texture and Formation Environment (Intrusive vs. Extrusive)
- Intrusive Igneous Rocks:
- Form underground from magma cooling deep below the surface of the Earth.
- Extended cooling time allows individual mineral crystals ample time to grow larger.
- Mineral crystals reach sizes visible to the naked eye, appearing as distinct colored grains in hand samples and larger structures under microscopes.
- Growth analogy: Like a tree growing larger given more time, mineral crystals grow larger given longer cooling times.
- Extrusive Igneous Rocks:
- Form on the surface of the Earth from lava that cools extremely rapidly.
- Rapid cooling prevents large crystal growth, keeping mineral grains small or fine-grained.
- Visual appearance typically presents a solid uniform color matrix, sometimes with microscopic specks.
- Tectonic environment example: At divergent boundaries where two plates pull apart, magma ascends and erupts as lava on the ocean floor.
Classification System 2: Chemical Composition (Felsic vs. Mafic)
- Igneous rocks are classified based on the dominance of specific chemical elements, which impart distinct colors to minerals.
- Felsic Igneous Rocks: Rich in lighter chemical elements (e.g., silicon and aluminum) and lighter in overall color.
- Crustal compositional distinctions:
- Continental Crust: Composed mostly of lighter, less dense felsic igneous rocks.
- Oceanic Crust: Composed of denser mafic igneous rocks.
Dual Classification Rule:
- Defining any igneous rock completely requires specifying both categories simultaneously:
1. Texture/Origin: Either intrusive or extrusive.
2. Composition: Either felsic or mafic.

Subsurface Physical Conditions and Phase Diagrams

Subsurface pressure and temperature behavior:
- Both temperature and hydrostatic/lithostatic pressure increase as depth increases below the surface of the Earth.
- High pressure inhibits rock melting; increased depth and temperature alone do not guarantee melting without pressure changes.
Pressure and temperature relationships (Boiling vs. Melting):
- Water boiling analogy:
- At standard sea level pressure, water boils at $100^\circ\text{C}$ ( $212^\circ\text{F}$ ).
- As elevation/altitude increases, atmospheric pressure decreases, reducing the required boiling temperature of water.
- Rock melting relationship:
- As pressure increases, the temperature required to heat and melt rock also increases.
- As pressure decreases, the required melting temperature decreases.
Geological Phase Diagram Conventions:
- Y-axes display two parameters: Pressure (in kilobars) and Depth (in kilometers).
- Zero value ( $0\,\text{km}$ depth) sits at the top of the Y-axis representing the Earth's surface, reading from top to bottom into deeper earth layers.
- X-axis displays Temperature in degrees Celsius ( $^\circ\text{C}$ ), ranging from $0^\circ\text{C}$ up to $3000^\circ\text{C}$ .
- Stability fields:
- Solid field (purple domain): Represents conditions where rock remains solid.
- Liquid field: Represents conditions where rock melts into magma.
- Solidus line: Boundary line separating the solid stability field from the liquid stability field.

Mechanisms of Magma Generation (Rock Melting)

Mechanism 1: Thermal Melting (Direct Heating)
- Melting rock by directly increasing its temperature at constant pressure.
- Everyday analogy: Melting chocolate in a pot on a stove over a flame.
- Phase diagram scenario: A solid rock situated at a depth of $200\,\text{km}$ , under $70\,\text{kb}$ of pressure, and at a temperature below $1000^\circ\text{C}$ .
- Process: Heat is added while depth and pressure remain constant.
- Diagram movement: The state point shifts horizontally to the right across the solidus line into the liquid stability field.
Mechanism 2: Decompression Melting
- Melting caused by a decrease in pressure without requiring an increase in temperature.
- Operational modes:
1. Pressure decreases while the rock remains in a fixed location.
2. The rock physically moves upward to a shallower depth where surrounding pressure is lower.
- Phase diagram scenario: Solid rock situated deep in the mantle at a depth of $550\,\text{km}$ under extreme pressure and temperatures exceeding $2000^\circ\text{C}$ . High pressure prevents the rock from melting despite high heat.
- Process: Rock physically rises toward the surface, lowering overburden pressure.
- Diagram movement: The state point shifts vertically upward on the diagram across the solidus line into the liquid stability field.
Mechanism 3: Flux Melting (Addition of Water or Volatiles)
- Melting induced by changing the melting temperature of rock through the addition of fluxing agents.
- Volatiles involved: Water ( $H_2O$ ) and gas-phase volatiles like carbon dioxide ( $CO_2$ ).
- Mechanism behavior:
- Does NOT change the actual temperature of the rock.
- Does NOT change the actual pressure of the rock.
- Lowers the solidus curve (the temperature threshold at which the rock melts).
- Diagram movement: The rock's positional point stays completely stationary while the solidus line bends and shifts to the left past the point, placing the rock into the liquid field.

Questions & Discussion

Question: Why is a mineral composed entirely of carbon not classified as a carbonate?
Answer: A mineral consisting of a single element (such as pure carbon or pure sulfur $S$ ) is classified as a native element. Carbonates require a specific molecular combination of carbon and oxygen ( $CO_3$ complex).
Question: How are rock samples categorized based on visual appearance into intrusive and extrusive groups?
Answer: Samples displaying multiple visible distinct mineral colors (such as Rock 1 and Rock 4) are intrusive because coarse grains indicate slow cooling. Samples displaying a single, mostly solid color (such as Rock 2 and Rock 3) are extrusive because fine grains indicate fast surface cooling.
Question: What occurs during decompression melting as rock moves relative to sea level?
Answer: Decompression melting occurs when a hot solid mantle rock ascends toward the Earth's surface. As the rock rises upward, overburden pressure decreases. Pressure increases with depth below surface/sea level and decreases closer to the surface.