Minerals and Rocks
Learning Objectives
- Identify common rock-forming minerals and their properties.
- Understand the rock cycle and the formation of igneous, sedimentary, and metamorphic rocks.
- Explain the engineering significance of intact rock versus rock mass.
- Define index properties like porosity and void ratio.
Minerals
The fundamental building blocks of the Earth's crust and all geological materials.
Mineral
A naturally occurring, inorganic solid with a definite chemical composition and a crystalline structure.
Rock-Forming Minerals
Over 4,000 minerals exist, but only a few dozen make up the bulk of the Earth's crust. These are the rock-forming minerals.
Common Rock-Forming Minerals
- Quartz: Hard (), resistant to weathering. A major component of granite and sandstone.
- Feldspar: The most abundant group (50% of the crust). Weathers to clay (kaolinite) in humid conditions.
- Mica: Platy structure (sheet silicates) with perfect cleavage. Biotite (black) and Muscovite (white). Can reduce shear strength.
- Calcite: Primary component of limestone (). Soluble in weak acid, leading to karst topography (caves, sinkholes).
Chemical Classes of Minerals
While physical properties identify minerals in the field, their chemical composition determines their classification and behavior:
Chemical Classes of Minerals
- Silicates: The most abundant class, forming 90% of the Earth's crust (e.g., Quartz, Feldspar, Mica). Based on the silicon-oxygen tetrahedron ().
- Carbonates: Important for cement and concrete, often soluble (e.g., Calcite , Dolomite).
- Sulfates: High solubility and potential for expansive reactions in soil (e.g., Gypsum , Anhydrite).
- Oxides: Principal sources of metal ores (e.g., Hematite , Magnetite).
- Sulfides: Often associated with acid mine drainage when exposed to oxygen and water (e.g., Pyrite ).
Physical Properties
Geologists identify minerals by their physical properties. Key properties include:
Key Physical Properties of Minerals
- Cleavage: The tendency of a mineral to break along flat planar surfaces (e.g., Mica has perfect basal cleavage).
- Fracture: The way a mineral breaks other than along a cleavage plane (e.g., Quartz has conchoidal fracture).
- Specific Gravity: The ratio of a mineral's weight to the weight of an equal volume of water. Typical silicates are 2.6 to 2.8, while metallic minerals are much heavier.
- Luster: How light is reflected from the surface (e.g., metallic, glassy, dull).
Mohs Hardness Scale
Mohs Hardness Scale Steps
- Talc (Softest, greasy feel)
- Gypsum (Fingernail scratch)
- Calcite (Penny scratch)
- Fluorite
- Apatite (Knife scratch)
- Orthoclase (Glass scratch)
- Quartz
- Topaz
- Corundum (Ruby/Sapphire)
- Diamond (Hardest natural substance)
Rocks
Interactive Simulation
Interact with the diagram below to understand the dynamic transitions of the Rock Cycle.
Select a stage in the rock cycle diagram to see details about its formation and properties.
Aggregates of minerals that form the solid Earth.
Rock
A consolidated mixture of minerals. A rock can be monomineralic (e.g., pure limestone) or polymineralic (e.g., granite).
The Rock Cycle describes the dynamic transitions between the three main rock types through geological time.
1. Igneous Rocks
Formed from the cooling and solidification of magma or lava.
Types of Igneous Rocks
- Intrusive (Plutonic): Slow cooling deep underground allows large crystals to grow (e.g., Granite, Gabbro). High strength, good for foundations.
- Extrusive (Volcanic): Fast cooling on the surface results in fine-grained or glassy texture (e.g., Basalt, Obsidian, Pumice). Basalt is often fractured (columnar jointing).
Bowen's Reaction Series
The crystallization sequence of magma.
Developed by Norman L. Bowen, this sequence explains the specific order in which silicate minerals crystallize from cooling magma. It is crucial for engineers because the order of crystallization is inversely related to a mineral's stability at the Earth's surface (weathering resistance).
Bowen's Reaction Series Sequences
- Discontinuous Series: Minerals form distinct crystal structures at specific temperatures (Olivine Pyroxene Amphibole Biotite). Olivine forms at the highest temperatures and is the least stable at surface conditions (weathers rapidly).
- Continuous Series: Plagioclase feldspar smoothly transitions from calcium-rich (high temperature) to sodium-rich (lower temperature).
- Residual Phases: Potassium Feldspar, Muscovite, and Quartz form last at the lowest temperatures. Quartz is highly stable and extremely resistant to chemical weathering, dominating most beach sands and sandstones.
2. Sedimentary Rocks
Formed by the accumulation and lithification (compaction and cementation) of sediments. The classification is primarily based on grain size and composition.
Types of Sedimentary Rocks
- Clastic: From fragments of other rocks. Gravel () forms Conglomerate or Breccia; sand () forms Sandstone; silt () forms Siltstone; clay () forms Shale or Mudstone. Shale is typically weak and highly prone to landslides along bedding planes.
- Chemical: Precipitated from solution (e.g., Limestone , Gypsum , Halite ). Limestone is soluble and can form massive subterranean voids (caves).
- Organic: From remains of plants and animals (e.g., Coal, Chalk).
3. Metamorphic Rocks
Formed by the alteration of pre-existing rocks due to heat, pressure, and chemically active fluids without melting.
Types of Metamorphic Rocks
- Foliated: Layered structure due to directed pressure. Low-grade Slate has excellent rock cleavage, Phyllite has a glossy sheen, Schist contains visible platy mica with weak cleavage planes, and high-grade Gneiss has distinct light and dark bands.
- Non-Foliated: Massive structure without planar fabrics. Marble is metamorphosed limestone and remains relatively soft and soluble, while Quartzite is metamorphosed quartz sandstone and is extremely hard and abrasive.
Engineering Properties
Understanding the physical behavior of geological materials.
Intact Rock vs. Rock Mass
A fundamental concept in engineering geology is the stark difference between the small-scale and large-scale behavior of rock.
Intact Rock
Rock Mass
Design Implications
Civil engineers must almost never use intact rock laboratory strength values directly for the design of large structures like dams or tunnels. The design must be based on the significantly reduced strength of the overall Rock Mass.
Specific Engineering Considerations
Certain rock and mineral characteristics pose severe challenges to engineering materials.
Critical Engineering Material Challenges
- Alkali-Silica Reactivity (ASR): A deleterious swelling reaction that occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline (amorphous) silica found in many common aggregates, such as opal, chalcedony, chert, and strained quartz. ASR causes severe internal expansion and extensive cracking in concrete structures, severely reducing their lifespan.
- Slake Durability: The resistance of a weak rock (such as shale, mudstone, or siltstone) to degradation when subjected to alternating cycles of wetting and drying. Rocks with a low Slake Durability Index rapidly break down into soil-like material upon exposure to the atmosphere. This is a critical consideration for slope stability in road cuts and the suitability of these materials for use as embankment fill. If a shale with low slake durability is used as fill, it will degrade and cause massive settlement.
Index Properties
Important volumetric and weight properties include:
Important Index Properties
- Porosity (): Volume of voids / Total volume. Affects water storage.
- Void Ratio (): Volume of voids / Volume of solids.
- Specific Gravity (): Ratio of the density of the solid to the density of water.
- Permeability: Ability to transmit fluids.
Porosity and Void Ratio
Formulas relating porosity () and void ratio ().
Variables
| Symbol | Description | Unit |
|---|---|---|
| Porosity | dimensionless | |
| Void Ratio | dimensionless |
- Minerals are the naturally occurring, inorganic solid building blocks of rocks. The Mohs Scale determines relative hardness.
- Igneous Rocks form from cooling magma/lava; intrusive types (Granite) are generally excellent for foundations.
- Sedimentary Rocks are classified primarily by grain size (sandstone, shale); they are layered, often porous, and susceptible to chemical weathering (e.g., Limestone).
- Metamorphic Rocks are altered by heat and pressure; their foliation planes (in Slate, Schist) dictate directional strength.
- Porosity and Void Ratio are fundamental properties defining a rock's ability to store fluids.
- There is a massive distinction between the high strength of Intact Rock tested in a lab and the much lower strength of the fractured Rock Mass encountered in the field.
- Geological discontinuities govern the actual engineering behavior of the ground.