Rocks and Minerals
Notes by Elly Keneddy Nalitso · 0708838163 / 0779031329
Learning outcomes
By the end of this chapter, you should be able to:
- Explain how igneous, sedimentary and metamorphic rocks are formed, that they are made of minerals, and that some minerals are useful.
- Identify the physical properties of rocks and distinguish between the different types of rocks.
- Explain the process of weathering and how weathered rock particles form the basis of soil.
Introduction
- The earth we live in is made up of rocks. We use rocks for different purposes such as construction of houses and roads.
- Rocks are naturally occurring solid aggregates of minerals, mineraloids, or organic material.
- They form the Earth's outer solid layer, the lithosphere, and are classified into three main types based on their formation processes.
- Rocks are composed of various components that determine their characteristics and classification.
Components of rocks
- Minerals
- Naturally occurring, inorganic solids with a specific chemical composition and crystalline structure. Common minerals in rocks include quartz, feldspar, mica, calcite and olivine.
- Mineraloids
- Naturally occurring substances that do not have a crystalline structure, which distinguishes them from true minerals. Examples: opal and volcanic glass (obsidian).
- Organic material
- In some sedimentary rocks, organic materials such as plant debris, shells and other biological remnants can be significant components. Coal, for example, is a sedimentary rock composed primarily of plant material.
- Matrix
- The finer-grained material in which larger grains, crystals or clasts are embedded. In sedimentary rocks the matrix can consist of fine particles like clay or silt.
- Cement
- In sedimentary rocks, the mineral matter that binds the grains together. Common cementing agents: silica, calcite and iron oxides.
- Clasts or grains
- Fragments of pre-existing rocks or minerals incorporated into a sedimentary rock. Clasts range in size from clay particles to large boulders (gravel, sand, silt, clay).
Types of rocks
Rocks are classified into three main types based on their formation processes: igneous, sedimentary and metamorphic.
Igneous rocks
Igneous rocks form from the cooling and solidification of molten rock material called magma (beneath the Earth's surface) or lava (at the surface). They are of two types:
Intrusive (plutonic) igneous rocks form from magma that cools slowly beneath the Earth's surface, allowing large crystals to develop, so the rock is coarse-grained. Example: granite, a coarse-grained rock composed mainly of quartz, feldspar and mica.
Extrusive (volcanic) igneous rocks form from magma that cools quickly at or near the surface, giving fine-grained or glassy textures with little crystal growth. Example: basalt, a fine-grained rock rich in iron and magnesium.
Types, texture and composition
| Silica content | Extrusive (volcanic, aphanitic texture) | Intrusive (plutonic, phaneritic texture) |
|---|---|---|
| Felsic (highest silica) | Rhyolite | Granite |
| Intermediate | Andesite | Diorite |
| Mafic (lowest silica) | Basalt | Gabbro |
Characteristics
- Texture: coarse-grained (phaneritic), fine-grained (aphanitic), glassy, or porphyritic (large crystals in a fine matrix).
- Composition: typically silicate minerals such as quartz, feldspar, mica, pyroxene and olivine.
- Color: varies widely; light (felsic), intermediate, or dark (mafic) depending on mineral content.
- Formation: cooling and solidification of magma or lava.
Examples: basalt, obsidian, pumice, rhyolite, andesite, diorite, gabbro, tuff.
Sedimentary rocks
Sedimentary rocks form from the accumulation and lithification of sediment, which can include fragments of other rocks, minerals and organic material. The process involves weathering, transportation, deposition and cementation. These consolidate into a large hard rock under the pressure of overlying deposits. Materials are eroded and transported by water, ice and wind.
- Eroded dissolved minerals are normally deposited at the bottom of water bodies.
- Remains of organisms settle at the seabed.
- Plant remains buried on land become compressed, forming layers.
- Over a long period under pressure, these consolidate into a hard rocky mass.
Formation
- Erosion: particles are carried away by wind or water.
- Sedimentation / deposition: eroded sediments end up in water and settle as loosely packed sediment.
- Compaction: more layers pile up and press down the lower layers; further compaction forces out water.
- Cementation: dissolved minerals (e.g. salt crystals) glue the layers together. The rock mass formed is sedimentary.
Characteristics
- Texture: from coarse-grained (conglomerate) to fine-grained (shale); clastic (fragmental) or non-clastic (chemical or organic).
- Composition: often quartz, calcite, clay minerals and organic material.
- Color: varies with composition; often shades of brown, gray, red and yellow.
- Layers: typically show distinct layers or strata.
- Formation: accumulation and lithification of sediment.
Examples of clastic sedimentary rocks: conglomerate, sandstone, shale, breccia, siltstone, mudstone.
Categories
Mechanical sedimentary rocks form from mechanical weathering debris, which is compacted and compressed into rock. Example: sandstone, composed of sand-sized particles, typically quartz.
Chemical sedimentary rocks form from precipitation of dissolved minerals from water at the bottom of water bodies; the precipitates accumulate and are compressed into rock. Example: limestone, primarily calcite, often formed from the remains of marine organisms.
Organic sedimentary rocks form from the accumulation of plant or animal debris on land, compressed over a long time into a hard rocky mass. Example: coal, formed from accumulated plant material, usually in swamps.
Metamorphic rocks
Metamorphic rocks form from the alteration of existing rocks (igneous, sedimentary or other metamorphic rocks) through high pressure, high temperature or chemically active fluids. These cause changes in mineral composition and texture without the rock melting.
Foliated metamorphic rocks have a layered or banded appearance due to alignment of mineral grains under directed pressure. Example: schist, with well-developed foliation, often containing mica. Others: slate, phyllite, gneiss.
Non-foliated metamorphic rocks do not show a banded or layered appearance. Example: marble, formed from limestone and consisting mostly of recrystallized calcite. Others: quartzite, hornfels.
Types of metamorphism shown in the notes: regional (low to high grade, producing foliated rocks) and contact (near hot magma, producing non-foliated rocks).
Some metamorphic rocks and their parent rocks
| Metamorphic rock | Characteristics | Parent rock(s) |
|---|---|---|
| Quartzite | Mostly white to pale grey; grainy surface with a sandpaper texture. | Sandstone and quartz |
| Marble | Medium-grained interlocking crystals with no alignment. | Limestone |
| Slate | Fine-grained interlocking crystals that lie flat in the same direction. | Mudstone |
| Schist | Medium-grained interlocking crystals; shiny appearance. | Shale |
| Gneiss | Coarse-grained interlocking crystals; banded texture. | Granite |
Characteristics
- Texture: foliated (layered or banded) or non-foliated.
- Mineral composition: often new minerals formed under high pressure and temperature, such as garnet, kyanite and staurolite.
- Color: varies widely, influenced by the original rock and the minerals formed.
- Hardness and density: typically harder and denser than the precursor rocks.
- Formation: alteration of existing rocks under high pressure, high temperature or chemically active fluid.
Examples: slate, phyllite, schist, gneiss, skarn, novaculite, hornfels, soapstone, blueschist, eclogite, migmatite, serpentinite.
Each type of rock tells part of the geological history of the Earth, revealing processes such as volcanic activity, sedimentation and mountain-building.
Minerals
Minerals are naturally occurring solids with a definite chemical composition and crystal structure.
- Primary minerals: original components of rocks (feldspar, mica, etc.).
- Secondary minerals: formed from changes in primary minerals and rocks (clay minerals).
- Essential minerals: the chief constituents of rocks (feldspars, pyroxenes, micas, etc.).
- Accessory minerals: present in small quantities; their presence or absence does not alter the properties of the rock (tourmaline, magnetite).
Examples of minerals and their uses
| Mineral | Uses |
|---|---|
| Quartz | Sand with quartz pebbles is used to manufacture glass. Used to make silicon semiconductors in computers. |
| Feldspar | Glassmaking and ceramics. Filler and extender in paints, plastics and rubber. |
| Mica | Making toothpastes and ceramics. Insulator in various electronics. |
| Muscovite | Used as glass because it is transparent. Manufacture of fireproofing and insulating materials. |
| Calcium carbonate | Food supplement, as a source of calcium for strong bones and teeth. Used to make antacid tablets. |
Other minerals pictured: beryl, cassiterite (tin ore), citrine, cuprite, goethite, hematite (iron ore), howlite, jaspillite, peridotite with phlogopite, prasiolite, pyrite, sodalite, black tourmaline (schorl) and turquoise.
The rock cycle
The rock cycle is a continuous process through which rocks are transformed from one type to another over geological time. It is driven by Earth's internal heat, pressure, and surface processes such as weathering and erosion.
- Weathering and erosion: igneous rocks on the surface are broken into small particles by wind, water and other natural forces.
- Sedimentary rock formation: particles are transported and deposited in layers, usually in water. The layers build up, then are compressed and cemented into sedimentary rocks like sandstone, limestone and shale.
- Metamorphism: when sedimentary rocks are buried deep in the crust, high heat and pressure change them into metamorphic rocks like marble and slate.
- Melting: metamorphic rocks can melt under extreme heat, forming magma. The magma cools and solidifies into igneous rock, continuing the cycle.
Note: igneous and metamorphic rocks can also be weathered into sediment, and igneous rocks can be metamorphosed or melted directly.
Weathering
Weathering is the disintegration of rocks into small particles. It is important because it results in the formation of soil.
Physical (mechanical) weathering
Rocks are broken down into smaller pieces without producing any new substances. Causes include heat, cold, water and wind.
- Freezing of water
- Liquid water enters joints and holes in a rock. As temperatures drop to 32 °F (0 °C) and below, it freezes and expands by about 10 percent. This pushes the cracks outward and causes the rock to break.
- Alternate wetting and drying
- In dry weather, clays shrink and form deep, wide cracks; on wetting they swell. This repeated swelling and shrinking makes clay-rich rocks loose and they eventually break.
- Action of glaciers
- In cold regions snow accumulates and turns into ice sheets. Glaciers move with changes in temperature or gradient, exert tremendous pressure on the rock beneath them and carry away loose material.
- Action of wind
- Wind erodes and transports material. When laden with fine sand, silt or clay, it abrades rocks, which eventually break down under its force.
Chemical weathering
Decomposition of rocks and minerals by chemical processes. It is the most important process for soil formation.
- It takes place mainly at the surface of rocks and minerals, with disappearance of certain minerals and formation of secondary products (new materials).
- It becomes more effective as surface area increases: the smaller the fragments, the greater the surface area per unit volume available for reaction.
- Carbonation
- Carbon dioxide dissolved in water forms carbonic acid, which attacks many rocks and minerals and brings them into solution. Carbonated water etches some rocks, especially limestone; removal of the cement that holds sand particles together leads to disintegration.
- Oxidation
- Addition and combination of oxygen to minerals, from oxygen dissolved in soil water and from the atmosphere. It is more active in the presence of moisture and produces hydrated oxides, e.g. in minerals containing Fe and Mg.
- Hydration
- Chemical combination of water molecules with a substance or mineral, changing its structure. Soil-forming minerals contain no water and hydrate when exposed to humid conditions. They swell, increase in volume, lose their luster and become soft, so they break down.
Biological weathering
Living agents cause both decomposition and disintegration of rocks and minerals, and the activity of this life is largely controlled by the prevailing environment. For example, seeds collect in cracks, grow into plants, and eventually the tree's roots exert pressure on the rock and split it.
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