Understanding mineral composition is crucial in the field of geology. It involves knowing the elements and ions that make up a mineral, as well as their bonding and saturation properties. One way to classify minerals is based on their composition, grouping them into classes with similar properties.
One example of a classification system based on mineral composition is the tungstate classes. Tungstates are minerals that contain the tungstate ion (WO4)2-. They have a unique crystal structure known as scheelite-type structure, which consists of face-sharing octahedra. Some examples of tungstates include nickel tungstate and calcium tungstate.
Another example of mineral classes based on composition is oxides. Oxides are minerals that contain oxygen and one or more other elements. They can be further divided into subcategories such as simple oxides, hydroxides, and multiple oxides. Lead oxide is an example of a simple oxide.
Sulfates are another class of minerals based on composition. Sulfates contain sulfate ions (SO42-) and can be subdivided into categories such as anhydrous sulfates and hydrated sulfates. An example member of this class is aluminum telluride.
Classification based on mineral composition helps geologists identify new minerals by comparing their properties to those already identified within a specific class. This system also aids in understanding how different types of minerals form under various geological conditions.
Chemical and internal structure of minerals for classification purposes
Crystal structure and chemical composition are two essential features used in the classification of minerals. The arrangement of atoms and cations in their crystal structures determines the chemical formulae of minerals. Understanding these aspects is crucial for geologists to identify, name, and classify minerals accurately.
Chemical Composition
The chemical composition of a mineral refers to the elements that make up its structure. It determines the physical properties of a mineral such as color, hardness, density, and luster. For instance, quartz has a silicon dioxide (SiO2) chemical formula that gives it a unique crystalline structure responsible for its glassy appearance. The calcite group is another example of minerals classified based on their chemical composition. Calcite (CaCO3), aragonite (CaCO3), and vaterite (CaCO3) have the same basic chemistry but differ in their crystal structures.
Internal Structure
The internal structure of a mineral refers to how individual crystals are arranged within it. Crystal system describes this feature based on symmetry elements such as rotation axes or mirror planes that can be used to transform one part of the crystal into another identical part. For instance, halite (NaCl) belongs to the cubic system because it has three mutually perpendicular four-fold axes that pass through its center.
Classification Based on Chemical Formulae
Minerals can be classified into groups based on their chemical formulae. Native elements such as gold (Au), silver (Ag), copper (Cu), sulfur (S), diamond (C) are examples of minerals classified by their elemental composition rather than by their crystal structure or other physical properties. Metallic cations such as pyrite FeS2, sphalerite ZnS, chalcopyrite CuFeS2 belong to sulfide group based on similar anionic complexes formed with sulfur ions.
Calcium carbonate is another compound used in mineral classification. The calcite group and dolomite group are examples of minerals classified based on their chemical composition. Calcite (CaCO3) has a trigonal crystal structure, while dolomite (CaMg(CO3)2) belongs to the hexagonal crystal system.
Classifying minerals based on their composition
Silicate Minerals: The Most Common Type of Mineral
Classifying minerals based on their mineral composition is a common practice in geology. There are various types of minerals, but the most common type is silicate minerals. Silicate minerals are composed of silicate tetrahedra, which contain silicon and oxygen atoms. They make up over 90% of the Earth’s crust and can be found in rocks such as granite, basalt, and gabbro.
Silicate minerals can be further subdivided into groups based on their chemical composition. For instance, feldspar is a group of aluminosilicates that contain potassium or sodium ions. Quartz is another group of silicates that contains only silicon and oxygen atoms.
Ferromagnesian Minerals: Containing Iron and Other Metals
Another type of mineral classified by its composition is ferromagnesian minerals. These minerals contain iron and other metals or semimetals such as magnesium, calcium, aluminum, or titanium. Ferromagnesian minerals are usually dark-colored and heavy compared to light-colored silicate minerals.
Ferromagnesian minerals include olivine, pyroxene, amphibole, biotite mica, and hornblende. These minerals are commonly found in igneous rocks such as basalt and gabbro but can also occur in metamorphic rocks like schist.
Carbonate Minerals: Composed of Carbonate Groups
Carbonate minerals are another group that can be classified based on their mineral composition. They are composed of carbonate groups (CO3)2- combined with metal cations such as calcium (Ca), magnesium (Mg), iron (Fe), or zinc (Zn). Carbonate minerals include calcite, dolomite, siderite, rhodochrosite, malachite among others.
The carbonate class also includes the barite group which contains sulfate instead of carbonate groups but has similar crystal structures. Carbonate minerals are commonly found in sedimentary rocks such as limestone and dolomite.
Physical and chemical properties of igneous rocks for classification
Texture, color, and mineral composition are the primary factors that classify igneous rocks based on their physical and chemical properties. These properties are determined by the cooling and solidification process of igneous rocks. The chemical composition of igneous rocks also influences their melting points and the presence of water molecules.
Physical Properties
The texture of an igneous rock is a vital factor in its classification as it provides information about the cooling rate during formation. Rapid cooling leads to fine-grained textures, while slow cooling results in coarse-grained textures. For instance, basaltic lava cools quickly on the Earth’s surface, forming a fine-grained texture called aphanitic. In contrast, granite cools slowly beneath the Earth’s surface, forming a coarse-grained texture called phaneritic.
Color is another important physical property used to classify igneous rocks. It is usually determined by the minerals present in the rock. For example, dark-colored rocks such as basalt contain more iron and magnesium minerals than light-colored rocks such as granite.
Chemical Properties
The chemical composition of an igneous rock influences its melting point and whether or not it contains water molecules. The presence of water molecules affects how easily magma can flow through cracks in the Earth’s crust before solidifying into an igneous rock.
Igneous rocks with high silica content have higher melting points than those with low silica content because silica has strong bonds that require more heat to break apart. Basaltic magma has low silica content; therefore, it has lower melting points than rhyolitic magma that has high silica content.
Classification Based on Physical and Chemical Properties
Igneous rocks can be classified based on their physical properties such as texture and color or their mineral composition using Bowen’s Reaction Series. Bowen’s Reaction Series describes how minerals crystallize from magma at different temperatures.
Using this classification method, igneous rocks can be classified into two categories: intrusive and extrusive. Intrusive rocks form below the Earth’s surface and have coarse-grained textures due to slow cooling rates. Examples of intrusive rocks include granite and gabbro.
Extrusive rocks form on the Earth’s surface from lava that cools quickly, resulting in fine-grained textures. Examples of extrusive rocks include basalt and pumice.
Classifying igneous rocks by proportion of dark minerals (exercise)
Dark Minerals: The Key to Classifying Igneous Rocks
Classifying igneous rocks based on their mineral composition is a crucial exercise in geology. One of the most common methods of classification involves determining the proportion of dark minerals present in a rock sample. This ratio can provide valuable insights into the rock’s origin, cooling history, and potential uses.
Determining Proportions Using Specific Gravity
To determine the proportion of dark minerals in a rock sample, geologists often use specific gravity. This measurement compares the mass of the sample to that of an equal volume of water. Dark minerals such as aragonite and solid solution can form tetrahedra that bond with other minerals in the groundmass as the rock cools, leading to a change in form and the formation of lead veins containing cobalt and silver.
The ratio of dark to light minerals can vary widely depending on factors such as magma composition, cooling rate, and pressure conditions during formation. For example, basaltic rocks typically have high proportions of dark minerals such as pyroxene and olivine due to their rapid cooling from lava flows. In contrast, granitic rocks tend to have lower proportions of dark minerals due to their slower cooling from large plutonic bodies.
Implications for Rock Identification and Uses
Knowing the proportion of dark minerals present in an igneous rock can be helpful for identifying its type and potential uses. For instance, high proportions of pyroxene and plagioclase feldspar are characteristic features of gabbroic rocks like norite or troctolite that are commonly used as construction materials or decorative stones.
Similarly, volcanic rocks like basalt or andesite with high proportions of olivine or hornblende may contain valuable metals like nickel or copper that can be extracted through mining operations. By contrast, granitic rocks like granite or diorite with low proportions of dark minerals are often used as building materials or for decorative purposes due to their attractive colors and patterns.
Classification by grain size
Grains, or the individual crystals that make up a mineral, can vary greatly in size. The classification of minerals based on grain size is just one of the many ways to categorize them. In this section, we will explore the different types of grains and how they affect mineral classification.
Coarse-Grained vs Fine-Grained Minerals
Minerals with larger crystals are classified as coarse-grained while those with smaller crystals are fine-grained. Coarse-grained minerals have a more visible texture and are easier to identify than fine-grained minerals. They also tend to be more resistant to weathering and erosion due to their larger size.
The size of grains can be measured in angstroms, which is equivalent to 10^-10 meters. This small unit of measurement allows scientists to observe the differences between coarse and fine-grained minerals through streak tests. For example, olivine has a continuous spectrum of grain sizes while pyroxene has chains or sheets of atoms that affect its lattice structure.
Classification by Grain Size
Minerals are classified into different groups based on their grain size. Table 1 shows an example of how minerals can be grouped according to their grain size:
Table 1: Classification by Grain Size
Group | Grain Size Range | Examples
— | — | —
Ultramafic | >90% mafic minerals | Peridotite, dunite
Mafic | 45-90% mafic minerals | Basalt, gabbro
Intermediate | 25-60% mafic minerals | Andesite, diorite
Felsic | <15% mafic minerals | Granite, rhyolite
As shown in Table 1, ultramafic rocks contain mostly mafic (iron and magnesium-rich) minerals like peridotite and dunite while felsic rocks contain mostly non-mafic (silica-rich) minerals like granite and rhyolite. The classification of minerals based on grain size can be useful for identifying rocks and understanding their formation.
Grain Size and Lattice Structure
The size of grains also affects the lattice structure of a mineral. For example, iron atoms in pyroxene are arranged in chains while those in olivine are arranged in sheets. This difference in lattice structure affects the rate at which these minerals cool and solidify, resulting in different crystal sizes.
Volcanic rocks without phenocrysts and glassy volcanic rocks
Fine-Grained Volcanic Rocks Without Phenocrysts
Volcanic rocks without phenocrysts are fine-grained and lack large mineral crystals. These rocks, also known as aphanitic rocks, form when lava cools rapidly on the surface of the Earth. The rapid cooling does not allow time for large mineral crystals to form, resulting in a smooth texture that is difficult to see with the naked eye.
Aphanitic rocks can be classified based on their mineral composition. Felsic volcanic rocks, such as rhyolite, are rich in tectosilicates like quartz and feldspar. Intermediate volcanic rocks contain both tectosilicates and ferromagnesian silicates like pyroxene and amphibole. Mafic volcanic rocks, such as basalt, contain mostly ferromagnesian silicates.
One example of an aphanitic rock is pumice, which forms from frothy lava that traps gas bubbles during solidification. Pumice has a very low density and can float on water due to its high porosity.
Glassy Volcanic Rocks
Glassy volcanic rocks are formed from lava that cools too quickly for crystals to form. These rocks have a smooth texture similar to fine-grained volcanic rocks without phenocrysts but lack any visible minerals due to their glassy nature.
Obsidian is one of the most well-known glassy volcanic rocks and has been used by humans for thousands of years for tools and weapons due to its sharp edges when fractured. Obsidian forms when lava cools extremely quickly at the surface or just below it.
Porphyritic Texture
Both felsic and intermediate volcanic rocks can have a porphyritic texture with large plagioclase feldspar or pyroxene phenocrysts embedded in a fine-grained matrix. This occurs when magma cools slowly beneath the Earth’s surface before erupting onto the surface as lava. The slow cooling allows large mineral crystals to form before the remaining magma erupts.
Porphyritic texture can also be found in intrusive rocks like gabbro, which forms when magma cools slowly beneath the Earth’s surface and has a coarse-grained texture due to the larger mineral crystals that have formed.
Understanding classification based on mineral composition
Understanding classification based on mineral composition is crucial for geologists and scientists in identifying the different types of minerals and rocks. The chemical and internal structure of minerals play a significant role in their classification, as well as their physical and chemical properties.
Classifying minerals based on their composition involves identifying the dominant elements present, such as silicates, carbonates, sulfides, oxides, or halides. This information helps geologists understand the formation processes of minerals and how they relate to geological events.
The physical and chemical properties of igneous rocks are also essential for their classification. By examining factors such as grain size, proportion of dark minerals, and presence of phenocrysts or glassy textures, geologists can identify different types of igneous rocks. These classifications provide insight into the cooling history and origin of these rocks.
One exercise used in classifying igneous rocks is by proportion of dark minerals. This method separates rocks into categories such as ultramafic, mafic, intermediate, felsic or acid based on their mineral content. Understanding these classifications is critical for studying volcanic eruptions and predicting potential hazards associated with them.
Another way to classify rocks is by grain size. Rocks can be classified into coarse-grained (phaneritic) or fine-grained (aphanitic) depending on crystal sizes formed during cooling processes. These classifications help geologists understand the rate at which the rock cooled from its molten state.
Volcanic rocks without phenocrysts are classified as aphanitic while glassy volcanic rocks are classified by texture rather than mineral composition. Understanding these classifications helps scientists determine the type of eruption that produced them.
