Eleanor Finch
Minerals are naturally occurring substances that are usually solid, crystalline, stable at room temperature, and inorganic. There are almost 5000 known mineral species, but most rocks are formed from a few common minerals, known as rock-forming minerals. These include feldspars, quartz, amphiboles, micas, olivine, garnet, calcite, and pyroxenes. The presence of specific minerals in a rock can provide clues about its formation conditions, such as temperature, pressure, and composition. Rock-forming minerals determine the composition, classification, formation processes, properties, and economic significance of rocks. They exhibit a wide range of characteristics, including different colours, textures, hardness, and densities.
| Characteristics | Values |
|---|---|
| Definition | A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes. |
| Composition | Rock-forming minerals are typically silicate minerals, composed of silicon (Si) and oxygen (O), along with other elements such as aluminium (Al), calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), iron (Fe), and others. |
| Examples | Feldspars, quartz, amphiboles, micas, olivine, garnet, calcite, pyroxenes. |
| Mineral Associations | The presence of specific minerals in a rock can provide clues about the rock's formation conditions, such as temperature, pressure, and composition of the parent rock. For example, granite typically consists of quartz, feldspars, and mica. |
| Cleavage | The way minerals cleave or break can vary. Micas easily split along one plane of cleavage to form thin sheets, amphiboles exhibit two cleavage planes, and quartz does not exhibit cleavage due to its strong crystal structure. |
| Crystal Habits | Crystals can vary in shape and structure. Examples include acicular (needle-like), bladed (blade-like and flattened), botryoidal (grape-like masses), columnar (long and slender prisms), cubic (cube-shaped), and dendritic (tree-like). |
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What You'll Learn
- Feldspars: the most common types are orthoclase, plagioclase, and microcline
- Quartz: one of the most abundant minerals on Earth, with two forms: α-quartz and β-quartz
- Amphiboles: a significant cluster of inosilicate minerals that form prisms or needle-like crystals
- Micas: sheet silicates with near-perfect basal cleavage, easily splitting along one plane to form thin sheets
- Olivine: composed of silica, iron, and magnesium, and is typically green in colour
Feldspars: the most common types are orthoclase, plagioclase, and microcline
Feldspars are rock-forming minerals that are abundant in the Earth's crust. The three most common types of feldspars are orthoclase, plagioclase, and microcline.
Orthoclase is a common raw material for the manufacture of glass and ceramics such as porcelain and is also used in scouring powder. It is one of the ten defining minerals of the Mohs scale of mineral hardness, with a hardness of 6. Some intergrowths of orthoclase and albite are called moonstone when used in jewellery.
Plagioclase is very important for the classification of crystalline igneous rocks. It is one of the three key minerals, along with quartz and alkali feldspar, used to make the initial classification of the rock type. Plagioclase is usually white to greyish-white in colour, with a slight tendency for more calcium-rich samples to be darker. Alkali feldspar appears as the silica content becomes high.
Microcline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar that typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during the slow cooling of orthoclase and is more stable at lower temperatures than orthoclase. It may be clear, white, pale yellow, brick-red, or green, and is generally characterised by cross-hatch twinning. Amazon stone, or amazonite, is a green variety of microcline.
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Quartz: one of the most abundant minerals on Earth, with two forms: α-quartz and β-quartz
Quartz is one of the most abundant minerals on Earth, and it is a major component of many rocks, including granite, quartzite, and sandstone. It is known for its hardness, resistance to weathering, and wide range of colours and crystal forms. Quartz is a rock-forming mineral, and it is classified as a framework silicate.
Quartz is one of the eight elements that make up 98% of the Earth's crust. These eight elements are oxygen, silicon, aluminium, iron, magnesium, calcium, sodium, and potassium. The composition of minerals formed by igneous processes is directly controlled by the chemistry of the parent body. For example, a magma rich in iron and magnesium will form minerals such as olivine and pyroxene, which are found in basalt.
Quartz is found in granite, which is formed from magma richer in silicon. This type of magma will form more silica-rich minerals, such as feldspar and quartz. The mineral association of quartz, feldspars, and mica is characteristic of granite, and the rock typically has a coarse grain and a granitic texture.
Quartz has two forms: α-quartz and β-quartz. There is an abrupt transformation from α-quartz to β-quartz at 573°C (846 K). This transformation is followed by a substantial quantity shift, and ceramics or rocks that pass through this temperature limit can be induced to fracture.
Quartz does not have a weakness in its crystal structure, so it does not exhibit cleavage. When a quartz specimen is broken, it displays a conchoidal (shell-like) fracture.
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Amphiboles: a significant cluster of inosilicate minerals that form prisms or needle-like crystals
Amphiboles are a group of inosilicate minerals that form prisms or needle-like crystals. They are composed of double-chain SiO4 tetrahedra, linked at the vertices, and generally contain iron and/or magnesium ions in their structure. Amphiboles can be found in a variety of colours, including green, black, white, yellow, blue, brown, and colourless. They are an important component of many intermediate igneous and metamorphic rocks, and are often found in syenites and diorites.
The name "amphibole" comes from the Greek word "amfibolos", meaning "ambiguous" or "doubtful", reflecting the difficulty in distinguishing amphibole from pyroxene minerals, which have similar characteristics. Amphiboles exhibit oblique cleavage planes at around 120 degrees, while pyroxenes have cleavage angles of approximately 90 degrees. Additionally, amphiboles are less dense than pyroxenes.
Amphiboles are the primary constituent of amphibolites, which are metamorphic rocks composed primarily of amphibole, plagioclase, and other minerals. Amphibolites often form through the metamorphism of mafic igneous rocks such as basalt. The temperature during the formation of amphibolites can exceed 500 °C, and the pressure is typically less than 1.2 GPa.
Amphiboles of metamorphic origin include those developed in limestones by contact metamorphism, such as tremolite, and those formed by the alteration of other ferromagnesian minerals, such as hornblende, which is an alteration product of pyroxene. Hornblende is an important constituent of many igneous rocks and is particularly common in syenites and diorites.
In gneisses, amphiboles are the dominant minerals in the rocks' dark bands. While amphibole crystals can occur in pegmatites associated with diorite or syenite igneous rocks, they typically form as integral parts of larger rock masses, making it unusual to find them as isolated, well-developed crystals.
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Micas: sheet silicates with near-perfect basal cleavage, easily splitting along one plane to form thin sheets
Micas are sheet silicates with near-perfect basal cleavage, which means they can easily split along one plane to form thin sheets. This quality makes them distinct from other minerals and gives them a unique structure. Micas are an important component of granite, a common type of igneous rock. They are also found in other rock types, such as metamorphic rocks.
Micas are often associated with other minerals, such as quartz and feldspars, in the formation of granite. The mineral association of quartz, feldspars, and mica gives granite its characteristic coarse-grained and granitic texture. Micas are also found in other rock types, such as gneiss and schist, which are formed under high-temperature and high-pressure conditions.
The unique cleavage properties of micas make them useful in various applications. For example, the thin sheets of mica can be used as a insulating material in electronics and electrical equipment due to their ability to split into thin layers. Micas are also used in the cosmetics industry and as a filler or extender in paints and coatings.
Micas are a group of phyllosilicate minerals, which are sheet silicates with a layered structure. The layers of mica are held together by weak electrical forces, which allows for the perfect cleavage that is characteristic of this mineral group. The chemical composition of micas includes silica, aluminum, and other elements, giving them their distinctive properties.
In summary, micas are sheet silicates with near-perfect basal cleavage, allowing them to easily split along one plane to form thin sheets. They are an important component of various rock types, particularly granite, and have a range of applications due to their unique structure and properties.
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Olivine: composed of silica, iron, and magnesium, and is typically green in colour
Olivine is a mineral composed of silica, iron, and magnesium. It is a type of nesosilicate or orthosilicate, with the chemical formula X2SiO4, where X can stand for iron (Fe), magnesium (Mg), calcium (Ca), manganese (Mn), or nickel (Ni). These elements can substitute for one another, resulting in different proportions of iron and magnesium, even within a single crystal. The two end members of the Olivine series are fayalite (iron-rich) and forsterite (magnesium-rich). Fayalite was named after Fayal Island in the Azores, while forsterite was named after Johann Forster, a German naturalist who accompanied Captain James Cook on his voyages.
Olivine is typically green in colour, with a distinctive olive or pale yellow-green hue, and is sometimes translucent. Its colour is thought to be due to traces of nickel, although it can turn reddish due to the oxidation of iron. Olivine is named after its green colour, with "olivine" coming from the Latin word for olive tree, "oliva". The gemstone peridot, also known as chrysolite, is a type of olivine that is often used in jewellery. Peridot has been prized since ancient times, with Pliny mentioning it in 1500 B.C. and the gemstone being referenced in the Bible.
Olivine is an important rock-forming mineral, commonly found in mafic and ultramafic igneous rocks, as well as certain metamorphic rocks. It is one of the most common minerals by volume, constituting a significant portion of the Earth's upper mantle. Olivine occurs in rocks such as basalt, gabbro, and peridotite, and is believed to be the most abundant mineral in the Earth's upper mantle at shallower depths. The properties of olivine influence the rheology of the mantle and play a crucial role in driving plate tectonics.
Magnesium-rich olivine, or forsterite, crystallizes from magma that is rich in magnesium and low in silica. This type of olivine is found in mafic rocks such as gabbro and basalt, as well as in meteorites, on the Moon, and on Mars. Ultramafic rocks, such as peridotites, also contain substantial amounts of olivine, with dunite having an olivine content of over 90%. Iron-rich olivine, or fayalite, is less common but can be found in small amounts in rare granites and rhyolites.
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Frequently asked questions
Rock-forming minerals are fundamental components in the formation of rocks, playing a critical role in determining the composition, classification, formation processes, properties, and economic significance of rocks.
Some common rock-forming minerals include feldspars, quartz, amphiboles, micas, olivine, garnet, calcite, and pyroxenes.
Rock-forming minerals have specific chemical compositions that distinguish them from other minerals. They are typically silicate minerals, composed of silicon and oxygen, along with other elements such as aluminium, calcium, magnesium, and iron.
Studying rock-forming minerals is essential for understanding the Earth's geology and its history. They provide insights into the processes that shape the Earth's crust and its resources. Additionally, they are important for practical applications in various industries.
