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Amphibole Group

Amphibole (/ˈæmfəboʊl/) is a group of inosilicate minerals, forming prism or needlelike crystals,[1] composed of double chain SiO4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Amp.[2] Amphiboles can be green, black, colorless, white, yellow, blue, or brown. The International Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups.[3]

amphibole group

Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic.[4] In chemical composition and general characteristics they are similar to the pyroxenes. The chief differences from pyroxenes are that (i) amphiboles contain essential hydroxyl (OH) or halogen (F, Cl) and (ii) the basic structure is a double chain of tetrahedra (as opposed to the single chain structure of pyroxene). Most apparent, in hand specimens, is that amphiboles form oblique cleavage planes (at around 120 degrees), whereas pyroxenes have cleavage angles of approximately 90 degrees. Amphiboles are also specifically less dense than the corresponding pyroxenes.[5] Amphiboles are the primary constituent of amphibolites.[6]

Like pyroxenes, amphiboles are classified as inosilicate (chain silicate) minerals. However, the pyroxene structure is built around single chains of silica tetrahedra while amphiboles are built around double chains of silica tetrahedra. In other words, as with almost all silicate minerals, each silicon ion is surrounded by four oxygen ions. In amphiboles, some of the oxygen ions are shared between silicon ions to form a double chain structure as depicted below. These chains extend along the [001] axis of the crystal. One side of each chain has apical oxygen ions, shared by only one silicon ion, and pairs of double chains are bound to each other by metal ions that connect apical oxygen ions. The pairs of double chains have been likened to I-beams. Each I-beam is bonded to its neighbor by additional metal ions to form the complete crystal structure. Large gaps in the structure may be empty or partially filled by large metal ions, such as sodium, but remain points of weakness that help define the cleavage planes of the crystal.[7]

Amphiboles are minerals of either igneous or metamorphic origin. Amphiboles are more common in intermediate to felsic igneous rocks than in mafic igneous rocks,[8] because the higher silica and dissolved water content of the more evolved magmas favors formation of amphiboles rather than pyroxenes.[9] The highest amphibole content, around 20%, is found in andesites.[10] Hornblende is widespread in igneous and metamorphic rocks and is particularly common in syenites and diorites. Calcium is sometimes a constituent of naturally occurring amphiboles. Amphilotes of metamorphic origin include those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (such as hornblende as an alteration product of pyroxene).[11] Pseudomorphs of amphibole after pyroxene are known as uralite.[12]

Four of the amphibole minerals are commonly called asbestos. These are: anthophyllite, riebeckite, the cummingtonite/grunerite series, and the actinolite/tremolite series. The cummingtonite/grunerite series is often termed amosite or "brown asbestos", and riebeckite is known as crocidolite or "blue asbestos". These are generally called amphibole asbestos.[14] Mining, manufacture and prolonged use of these minerals can cause serious illnesses.[15][16]

Certain amphibole minerals form solid solution series, at least at elevated temperature. Ferrous iron usually substitutes freely for magnesium in amphiboles to form continuous solid solution series between magnesium-rich and iron-rich endmembers. These include the cummington (magnesium) to grunerite (iron) endmembers, where the dividing line is placed at 30% magnesium.[18]

In addition, the orthoamphiboles, anthophyllite and gedrite, which differ in their aluminium content, form a continuous solid solution at elevated temperature. As the amphibole cools, the two end members exsolve to form very thin layers (lamellae).[18]

There is not a continuous series between calcic clinoamphiboles, such as hornblende, and low-calcium amphiboles, such as orthoamphiboles or the cummingtonite-grunerite series. Compositions intermediate in calcium are almost nonexistent in nature.[20] However, there is a solid solution series between hornblende and tremolite-actinolite at elevated temperature. A miscibility gap exists at lower temperatures, and, as a result, hornblende often contains exsolution lamellae of grunerite.[21]

Glaucophane, crocidolite, riebeckite and arfvedsonite form a somewhat special group of alkali-amphiboles. The first two are blue fibrous minerals, with glaucophane occurring in blueschists and crocidolite (blue asbestos) in ironstone formations, both resulting from dynamo-metamorphic processes. The latter two are dark green minerals, which occur as original constituents of igneous rocks rich in sodium, such as nepheline-syenite and phonolite.[13][24]

Amphibole is an crucial institution of usually darkish-colored, inosilicate minerals, forming prism or needlelike crystals,composed of double chain SiO4 tetrahedra, connected at the vertices and normally containing ions of iron and/or magnesium in their systems. Amphiboles may be inexperienced, black, colorless, white, yellow, blue, or brown. The International Mineralogical association presently classifies amphiboles as a mineral supergroup, inside which might be businesses and several subgroups.

The minerals of the amphibole group crystallize in the orthorhombic, monoclinic, and triclinic systems, but the crystals of the different species are closely similar in many respects. Chemically they form a group parallel to the pyroxene group, being silicates with calcium, magnesium, and ferrous iron as important bases, and also with manganese and the alkalis. The amphiboles, however, contain hydroxyl. Certain molecules that are present in some varieties contain aluminum and ferric iron. The amphiboles and pyroxenes closely resemble one another and are distinguished by cleavage. The prismatic cleavage angle of amphiboles is about 56 and 124, while the pyroxene cleavage angle is about 87 and 93.

As the amphibole minerals primarily form as integral parts of larger rock masses, it is unusual to find them as isolated, large, well-developed crystals. Amphibole crystals, however, do occur in pegamatites associated with diorite or syenite igneous rocks. In igneous rocks, amphibole minerals are usually associated with the intermediate feldspar minerals, while in metamorphic rocks they are associated with mica minerals. In metamorphic rocks, amphibole minerals may themselves be altered into other iron-magnesium silicates such as chlorite, epidote and biotite.

Most of the common, rock-forming amphibole minerals have relatively little economic value on their own. Their sole economic contribution is being part of the dark mineral component of the granites, diorites and syenites widely used as decorative rock and building stones.

A relative lack of economic importance does not hold true for some less common members of the amphibole mineral group. Amphiboles include both the original jade gemstones exquisitely carved by Chinese and Maori artisans and many of the minerals collectively known as asbestos. There are actually two minerals known as jade in the gemstone trade. Nephrite (an amphibole) and jadeite (a pyroxene) are nearly indistinguishable by someone new to gems. Both exhibit the same green color and remarkable durability that has made jade a favored gemstone for jewelry and statues. Among the Aztec, jade was more valuable than gold.

It was the use of asbestos for automobile break linings and gaskets that first transformed asbestos into an important industrial commodity. Its durability and heat resistance soon led to asbestos becoming one of the more widely used materials for fireproofing, heat resistance and insulation. It was used in electric stoves and hotplates, to wrap pipes, insulate buildings and create fire protective gear. Incorporated into housing shingles, wallboard and floor tiles, asbestos soon became essential to the building industry until people started to become aware of its potential health risks. However, although they were the first minerals to be known as asbestos, the amphibole minerals only account for roughly a tenth of modern asbestos production. Most commercial asbestos comes from chrysotile, a fibrous variety of the mineral serpentine.

Chemically, the most significant differences between the pyroxene and amphibole groups are the addition of O- and OH-groups in the amphiboles, and the two groups' different silicate structures. The pyroxenes are single chain silicates, while the amphiboles are double chain silicates. In general, pyroxene crystals tend to be stubbier than the more elongated amphibole crystals, but the crystal shapes may be very similar in those amphiboles that formed from the alteration of pyroxenes. As a result, the only definitive way to distinguish the two groups is by the angle between cleavage faces on crystal fragments. On pyroxene fragments, the cleavage faces tend to meet at nearly right angles. In contrast, amphibole cleavage fragments have cleavage faces that meet at angles of nearly 60 degrees and 120 degrees. If you look down the long axis of a cleavage fragment, pyroxenes will tend to have rectangular cross-sections, while amphiboles will exhibit a diamond- or wedge-shaped pattern.

Samples of dark vitreous amphibole may be mistaken as having a metallic luster. This can cause them to be confused with magnetite or other black metallic minerals. However, none of these metallic minerals will exhibit the two well-developed cleavage directions present in amphibole minerals. Magnetite is also easily distinguished from amphibole by its magnetic character. 041b061a72

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