Minerals and Rocks - Geological Building Blocks
Minerals and rocks are the essential building blocks of the geosphere. Although there are over 3,000 species of minerals, only a few of them, such as quartz, feldspar, mica, amphibole, pyroxene, olivine and calcite, occur commonly as rock-forming minerals. Rocks are classified into three main types, igneous, sedimentary, and metamorphic, depending upon their mode of formation. Over geological time, rocks gradually are transformed from one type to another in what is termed the Rock Cycle. The origin of any particular rock can be determined by careful examination of its texture, composition, and internal structure, features that form the basis of rock identification and classification. |
- What are Minerals?
- Common Rock-forming Minerals
- What are Rocks?
- Igneous Rocks
- Sedimentary Rocks
- Metamorphic Rocks
- The Rock Cycle
What are Minerals?Minerals are the fundamental components of rocks. They are naturally occurring inorganic substances with a specific chemical composition and an orderly repeating atomic structure that defines a crystal structure. Silicate minerals are the most abundant components of rocks on the Earth's surface, making up over 90% by mass of the Earth's crust. The fundamental chemical building block of silicate minerals is the chemical compound silicon tetroxide, SiO4 (Figure 1). The common non-silicate minerals, which constitute less than 10% of the Earth's crust, include carbonates, oxides, sulphides, phosphates and salts. A few elements may occur in pure form. These include gold, silver, copper, bismuth, arsenic, lead, tellurium and carbon. Although 92 naturally occurring elements exist in nature, only eight of these are common in the rocks of the Earth's crust. Together, these eight elements make up more than 98% of the crust (Table 1).
Table 1. The eight most common elements in the Earth's crust (by mass).
Classification and IdentificationMinerals are classified according to their chemical composition. The physical properties of minerals, such as their hardness, lustre, colour, cleavage, fracture and relative density, can be used to identify minerals. These general characteristics are controlled mainly by their atomic structure (crystal structure). |
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Common Rock-forming Minerals |
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Quartz
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Figure 2: Quartz. |
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Plagioclase feldspar
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Figure 3: Plagioclase feldspar. |
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Alkali Feldspar
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Figure 4: Alkali feldspar. |
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Micas
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Figure 5: Biotite. |
Figure 6: Muscovite. |
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Amphiboles
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Figure 7: Hornblende. |
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Pyroxene
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Figure 8: Pyroxene. |
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Olivine
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| Figure 9: Olivine. |
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Calcite
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Figure 10: Calcite. |
What are Rocks?Rocks are naturally occurring aggregates of minerals, rock fragments or organic matter. The composition of a rock, as well as the appearance, shape, and arrangement of the grains or crystals within the rock (i.e. its texture), are the characteristics that reveal its process of formation. Based on their mode of formation, rocks are classified into three main types: igneous, sedimentary and metamorphic. Introduction to Rock IdentificationIn most cases, it is not possible to directly observe how rocks are formed. Therefore, it is necessary to rely on the distinctive features of a rock to infer its origin. Texture and mineral composition are two important characteristics that may help to confirm the origin of a rock.
The identification and classification of rocks is a skilled task that requires a broad understanding of geology and considerable experience. |
Igneous RocksIgneous rocks form when hot, molten rock (magma) cools and solidifies. The magma originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are sub-divided into either intrusive or extrusive rocks, depending upon where in the Earth the magma solidifies (Figure 11). |
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Figure 11: Various Forms of Igneous Rocks. |
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Characteristics of Intrusive Igneous Rocks |
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Granitic Rocks
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Figure 12 - Equigranular granite, i.e. mineral grains are of approximately the same size. |
Figure 13 - Inequigranular granite, containing large crystals (phenocrysts) of feldspar in a mixture of smaller crystals. |
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Characteristics of Extrusive Igneous Rocks |
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Lava
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Figure 14 - Coarse grain crystal tuff, containing mainly crystal fragments. |
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Tuff
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Figure 15 - Eutaxitic tuff (welded tuff), containing pumice fragments (fiamme) and glass shards that are flattened during its formation. |
Sedimentary RocksSedimentary rocks are formed from the eroded fragments of pre-existing rocks, or from the skeletal fragments of once-living plants or organisms. They accumulate in various environments on the Earth's surface. Sedimentary rocks commonly have distinctive layering or bedding. Sedimentary rocks are sub-divided into three groups, including clastic, biological and chemical.
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Characteristics of Sedimentary Rocks |
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Clastic Sedimentary Rocks
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Metamorphic RocksMetamorphic rocks are formed when a pre-existing rock is subject to high temperature, high pressure, hot and mineral-rich fluid, or a combination of these conditions. The original rocks could be igneous, sedimentary, or earlier metamorphic rocks. In the case of metamorphic rocks, some or all of the original minerals are replaced by new minerals, and the original textures are commonly masked due to the deformation (such as shearing and folding) that may accompany metamorphism. Metamorphic rocks are generally formed deep within the Earth, or where tectonic plates meet.
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Characteristics of Metamorphic Rocks |
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Schist and Phyllite
Marble
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The Rock CycleThe Rock Cycle (Figure 20) is a conceptual model that explains how geological processes acting on any one of the three main rock types - igneous, sedimentary and metamorphic - can change one rock type to another over geological time. Plate tectonics is the driving force of the Rock Cycle. In order to understand the Rock Cycle, it is important to understand the rock-forming processes.
The Rock Cycle can begin with any one of the three rock types. It is important to understand that a rock does not necessarily pass all the way through the Rock Cycle from igneous, to sedimentary, to metamorphic, and back to igneous rock again. For example, an igneous rock can become a metamorphic rock without reaching the Earth's surface, and without first becoming a sedimentary rock. Also, any type of rock can become a new rock of the same type. |
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Figure 20: The Rock Cycle.
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