Amphibole Mineral
The amphibole mineral is a mineral belonging to the inosilicate classification of minerals. They are known for and classified as such because of their structural configuration that leads to the formation of a prism or needle-like structures. The amphibole is made up of double-chain silica (SiO4) tetrahedra. In these tetrahedra, the two chains of silica are linked with one another at the vertices and typically contain ions of iron and/or magnesium in their structures. The Amphibole group of minerals is a supergroup according to the International Mineralogical Association as there are two more groups and several subgroups classified within it.
Naming of Amphibole
The amphibole meaning is derived from the ancient Greek language from the root word amphibolos. The term amphibolos means “double entendre” i.e. ambiguousness. Deriving amphibole meaning from the amphibolos term, the mineral amphibole was first used by René Just Haüy. With this nomenclature tremolite, actinolite and hornblende. Because of the composition and appearance of these minerals, these particular minerals were named amphibole meaning something that is showing dual and ambiguous properties.
Physical Properties of Amphibole
The following are the physical properties and chemical characteristics of the amphibole mineral:
Amphibole is made up of double chains of silica in a tetrahedral structure with both the chains being linked to each other at their vertices.
The crystalline structure of the amphibole contains ions of iron and/or magnesium within it.
The amphibole mineral can be found in different colors such as green, black, white, yellow, brown or some of its forms can be colorless as well.
The amphibole structure is known for the formation of two different types of crystals. The two types of crystals formed are either prismatic or needle-like in shape.
The amphibole chemical composition is essentially made up of hydroxyl or halogen groups within their crystal structure.
Although, there are similarities in-between certain properties of pyroxene and amphibole, the basic structure of the amphibole crystal is different from the pyroxene crystal as the pyroxene crystal is made up of a single chain of silicates and the amphibole is made up of double chains of silica.
Amphiboles form cleavage planes at around 120 degrees which is different from the pyroxenes as pyroxenes form the cleavage planes around 90 degrees.
The objects classified in the amphibole mineral group are also specifically less dense than the corresponding objects that belong to the pyroxene category.
Optically as well the properties of amphiboles are interesting because optically the amphiboles due to their unique amphibole structure display different colors when viewed or observed from different angles. They have a very strong pleochroism characteristic and a smaller angle of extinction on the plane of symmetry.
Characteristics of Amphibole
The minerals of the amphiboles can originate from either the igneous or metamorphic origin. The common forms of amphibole are present in both the intermediate to felsic igneous rocks rather than mafic igneous rocks because of the iron or magnesium ion content. The characteristics of amphibole found in the rocks and determined by the formation are given as follows:
The property of the amphibole structure to have double silica chains because of the higher silica content and higher dissolved water content found in the more evolved magmas. These are the conditions that favor the formation of amphiboles over the formation of pyroxenes.
Amphiboles are primary constituents of the amphibolites which also include actinolite, hornblende, plagioclase, etc.
Andesites are the ones that contain the highest amount of amphiboles that is 20% of the total composition.
Another one of the minerals included in the amphibole is the hornblende and is widespread in the igneous and metamorphic rocks and more prominently in the syenites and diorites.
The naturally occurring amphibole sometimes contains calcium as the main constituent.
The amphiboles formed include the metamorphic rocks the ones developed in limestones by the contact metamorphism and also the ones formed by the alteration of other ferromagnesian minerals like the hornblende which is a product of the pyroxene. After pyroxene, the pseudomorphs of amphibole are known as the uralite.
The amphibole formula that is found commonly in the representation of the minerals classified under it is RSi4O11 where R is the specific group present in different minerals.
Most common of the minerals among is the amphibole asbestos. Four such minerals are commonly known as amphibole asbestos. Those four are anthophyllite, riebeckite, cummingtonite/grunerite asbestos series and tremolite/actinolite asbestos series. The cummingtonite/grunerite are generally known as brown or amosite asbestos, whereas, the riebeckite mineral is known as blue asbestos. All of these are very commonly known as amphibole asbestos.
Use of Amphibole
The amphibole mineral finds its utility for a variety of purposes. Because of its coloring and the ability to exhibit different colors when being viewed from different angles, one of the most prominent uses of amphibole is in decorations. It can be simply shaped as desirable and then kept in the house as a decorative or interior designing item. Some of the observable uses are as paving stones, and as veneers or facings on the buildings. Another one of the uses of amphibole mineral includes its utility as crushed stone for activities such as road construction and railroad bed construction. This is vastly done near the sites where amphobiles are a common occurrence.
General contemplations
Amphiboles are tracked down chiefly in transformative and volcanic rocks; they happen in numerous transformative rocks, particularly those obtained from mafic volcanic rocks (those containing dim hued ferromagnesian minerals) and siliceous dolomites. Amphiboles likewise are significant constituents in an assortment of plutonic and volcanic molten rocks that reach in arrangement from granitic to gabbroic. Amphibole, from the Greek amphibolos, signifying "equivocal," was named by the popular French crystallographer and mineralogist René-Just Haüy (1801) in reference to the extraordinary assortment of creation and appearance shown by this mineral gathering. There are 5 significant gatherings of amphibole prompting 76 synthetically characterized end-part amphibole pieces as per the British mineralogist Bernard E. Leake. As a result of the wide scope of synthetic replacements allowable in the precious stone design, amphiboles can solidify in volcanic and transformative rocks with a wide scope of mass sciences. Normally amphiboles structure as long kaleidoscopic gems, emanating splashes, and asbestiform (sinewy) totals; in any case, without the guide of compound investigation, it is hard to megascopically distinguish everything except a couple of the more unmistakable end-part amphiboles. The mix of kaleidoscopic structure and two precious stone-formed headings of cleavage at around 56° and 124° is the demonstrative component of most individuals from the amphibole bunch.
Compound structure
The perplexing compound piece of individuals from the amphibole gathering can be communicated by the overall recipe A0–1B2C5T8O22(OH, F, Cl)2, where A = Na, K; B = Na, Zn, Li, Ca, Mn, Fe2+, Mg; C = Mg, Fe2+, Mn, Al, Fe3+, Ti, Zn, Cr; and T = Si, Al, Ti. Almost complete replacement might occur among sodium and calcium and among magnesium, ferrous iron, and manganese (Mn). There is restricted replacement between ferric iron and aluminum and among titanium and other C-type cations. Aluminum can to some extent substitute for silicon in the tetrahedral (T) site. Halfway replacement of fluorine (F), chlorine, and oxygen for hydroxyl (OH) in the hydroxyl site is additionally normal. The intricacy of the amphibole equation has led to various mineral names inside the amphibole bunch. In 1997 Leake introduced an exact classification of 76 names that include the compound variety inside this gathering. The mineral classification of the amphiboles is partitioned into four head regions dependent on B-bunch cation inhabitants:
(1) The iron-magnesium-manganese amphibole bunch,
(2) The calcic amphibole bunch,
(3) The sodic-calcic amphibole bunch,
(4) The sodic amphibole bunch.
Actual properties
Long kaleidoscopic, acicular, or sinewy gem propensity, Mohs hardness somewhere in the range of 5 and 6, and two bearings of cleavage converging at around 56° and 124° for the most part do the trick to recognize amphiboles close by examples. The particular gravity upsides of amphiboles range from around 2.9 to 3.6. Amphiboles yield water when warmed in a shut cylinder and wire with trouble in a fire. Their shading goes widely from dry to white, green, brown, dark, blue, or lavender and is connected with the arrangement, primarily the iron substance. Magnesium-rich amphiboles like anthophyllite, cummingtonite, and tremolite are luster or light in shading. The tremolite-ferro actinolite series goes from white to dull green with expanding iron substance. The finely stringy and enormous assortment of actinolite-tremolite known as nephrite jade reaches from green to dark. Normal hornblende is ordinarily dark. Glaucophane and riebeckite are typically blue. Anthophyllite is dim to different shades of green and brown. The cummingtonite-grunerite series happens in different shades of light brown. Sans iron assortments of tremolite containing manganese can have a lavender tone.
FAQs on Amphibole
1. What is the geological definition of amphibole?
Amphibole is a group of dark-coloured, rock-forming inosilicate minerals. Geologically, they are defined by their distinct crystal structure, which consists of double chains of silica tetrahedra (SiO₄). This internal arrangement is crucial as it dictates many of their physical properties, most notably their cleavage. They are common constituents of both igneous and metamorphic rocks.
2. What are the primary uses of amphibole minerals?
Due to their durability and appearance, amphibole minerals have several practical and decorative applications. Key uses include:
- Construction: Used as crushed stone for road construction and railway beds.
- Building Materials: Certain varieties are cut and polished for use as paving stones, dimension stone, and decorative facings or veneers on buildings.
- Asbestos: Some fibrous forms of amphibole, such as amosite and crocidolite, were historically used as asbestos, although this is now heavily regulated due to health risks.
- Gemstones: Nephrite, a variety of actinolite, is one of the two minerals known as jade and is highly valued as a gemstone.
3. How can you identify an amphibole mineral in a rock sample?
You can identify amphibole by observing a combination of its key physical properties:
- Cleavage: This is the most diagnostic feature. Amphiboles exhibit two directions of cleavage that intersect at approximately 56° and 124°.
- Crystal Habit: They typically form long, prismatic (like a prism) or acicular (needle-like) crystals.
- Colour: Most amphiboles are dark, commonly appearing dark green, brown, or black.
- Lustre: They have a vitreous (glassy) lustre on their cleavage surfaces.
- Hardness: Their hardness on the Mohs scale is typically between 5 and 6.
4. What is the general chemical formula for the amphibole group?
The chemical composition of amphiboles is complex and variable. The generalised chemical formula is AX₂Y₅Z₈O₂₂(OH)₂. In this formula, A, X, Y, and Z represent sites that can be occupied by a wide variety of cations, such as sodium (Na), calcium (Ca), magnesium (Mg), iron (Fe), and aluminium (Al). The presence of the hydroxyl (OH) group signifies that amphiboles are hydrous minerals, meaning they contain water in their structure.
5. Is amphibole a type of rock or a mineral? Explain the difference.
Amphibole is a mineral, not a rock. This is a common point of confusion. A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a specific crystalline structure. A rock, on the other hand, is an aggregate of one or more minerals. For example, the metamorphic rock amphibolite is a rock that is composed primarily of amphibole minerals, along with others like plagioclase feldspar.
6. How does amphibole's cleavage differ from pyroxene's, and why is this important?
The difference in cleavage is the primary way to distinguish amphiboles from pyroxenes, another group of chain silicate minerals. Amphibole, a double-chain silicate, has cleavage planes that intersect at angles of about 56° and 124°. In contrast, pyroxene, a single-chain silicate, has cleavage planes that intersect at nearly right angles (approximately 87° and 93°). This fundamental difference arises directly from their internal atomic structures and is a critical diagnostic tool for mineral identification in the field and lab.
7. What are some common examples of amphibole minerals?
The amphibole group includes many different mineral species. Some of the most common and important examples that a student might encounter are:
- Hornblende: The most common amphibole, typically black or dark green, found in many igneous and metamorphic rocks.
- Actinolite: A green amphibole common in metamorphic rocks. The fibrous variety, nephrite, is a type of jade.
- Tremolite: Often found in metamorphosed dolomites and limestones, it can be white, grey, or pale green.
- Glaucophane: A blue amphibole that forms under high-pressure, low-temperature metamorphic conditions.
8. What is the significance of the (OH) group in amphibole's chemical structure?
The presence of the hydroxyl (OH) group makes amphibole a hydrous mineral. This is significant because it indicates the mineral formed in a geological environment where water was present, such as during the metamorphism of wet basalt or in cooling magma bodies. The (OH) group also affects its stability; amphiboles tend to break down and convert to anhydrous minerals like pyroxene at very high temperatures, providing geologists with clues about the temperature and pressure conditions under which a rock formed.
