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 amphibole contains ions of iron and/or magnesium within it.
The amphibole mineral can be found in different colours such as green, black, white, yellow, brown or some of its forms can be colourless 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 colours 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.
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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 favour 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 colouring and the ability to exhibit different colours 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.