Amphibole

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Amphibole (tremolite) Tremolite-121232.jpg
Amphibole (tremolite)

Amphibole ( /ˈæmfɪˌbl/ ) is a group of inosilicate minerals, forming prism or needlelike crystals, [1] composed of double chain SiO
4
tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. 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. [2]

Contents

Mineralogy

Photomicrographs of a thin section containing an amphibole crystal; under cross-polarized light on the left, and plane-polarized light on the right. Thin section microscopy Siilinjarvi R636 6335 amphibole.jpg
Photomicrographs of a thin section containing an amphibole crystal; under cross-polarized light on the left, and plane-polarized light on the right.

Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic. [3] 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. [4] In optical characteristics, many amphiboles are distinguished by their stronger pleochroism and by the smaller angle of extinction (Z angle c) on the plane of symmetry.[ citation needed ] Amphiboles are the primary constituent of amphibolites. [5]

In rocks

Amphiboles are minerals of either igneous or metamorphic origin. Amphiboles are more common in intermediate to felsic igneous rocks than in mafic igneous rocks, [6] because the higher silica and dissolved water content of the more evolved magmas favors formation of amphiboles rather than pyroxenes. [7] The highest amphibole content, around 20%, is found in andesites. [8] 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). [9] Pseudomorphs of amphibole after pyroxene are known as uralite. [10]

History and etymology

The name amphibole (Ancient Greek ἀμφίβολος - amphíbolos literally meaning 'double entendre', implying ambiguousness) was used by René Just Haüy to include tremolite, actinolite and hornblende. The group was so named by Haüy in allusion to the protean variety, in composition and appearance, assumed by its minerals. This term has since been applied to the whole group. Numerous sub-species and varieties are distinguished, the more important of which are tabulated below in two series. The formulae of each will be seen to be built on the general double-chain silicate formula RSi4O11. [11]

Four of the amphibole minerals are among the minerals commonly called asbestos. These are: anthophyllite, riebeckite, cummingtonite/grunerite series, and actinolite/tremolite series. The cummingtonite/grunerite series is often termed amosite or brown asbestos; riebeckite is known as crocidolite or blue asbestos. These are generally called amphibole asbestos. [12] Mining, manufacture and prolonged use of these minerals can cause serious illnesses. [13] [14]

Mineral species

Chemical formula

Orthorhombic series

Monoclinic series

Descriptions

On account of the wide variations in chemical composition, the different members vary considerably in properties and general appearance.

Anthophyllite occurs as brownish, fibrous or lamellar masses with hornblende in mica-schist at Kongsberg in Norway and some other localities. An aluminous related species is known as gedrite and a deep green Russian variety containing little iron as kupfferite. [11]

Hornblende is an important constituent of many igneous rocks. It is also an important constituent of amphibolites formed by metamorphism of basalt. [15]

Actinolite is an important and common member of the monoclinic series, forming radiating groups of acicular crystals of a bright green or greyish-green color. It occurs frequently as a constituent of greenschists. The name (from Greek ἀκτίς, ἀκτῖνος/aktís, aktînos, a 'ray' and λίθος/líthos, a 'stone') is a translation of the old German word Strahlstein (radiated stone). [11] [16]

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. [11] [17]

Pargasite is a rare magnesium-rich variety of hornblende [10] with essential sodium, usually found in ultramafic rocks. For instance, it occurs in uncommon mantle xenoliths, carried up by kimberlite. It is hard, dense, black and usually automorphic, with a red-brown pleochroism in petrographic thin section. [18]

See also

Notes

  1. "Amphibole". Dictionary of Geology. Retrieved 2013-01-21.
  2. Mindat, Amphibole Supergroup
  3. Klein, Cornelis; Hurlbut, Cornelius S., Jr. (1993). Manual of mineralogy : (after James D. Dana) (21st ed.). New York: Wiley. p. 491. ISBN   047157452X.
  4. Klein & Hurlbut 1993, pp. 474-475,478,491.
  5. Klein & Hurlbut 1993, pp. 590.
  6. Peters, Stefan T. M.; Troll, Valentin R.; Weis, Franz A.; Dallai, Luigi; Chadwick, Jane P.; Schulz, Bernhard (2017-03-16). "Amphibole megacrysts as a probe into the deep plumbing system of Merapi volcano, Central Java, Indonesia". Contributions to Mineralogy and Petrology. 172 (4): 16. doi:10.1007/s00410-017-1338-0. ISSN   1432-0967.
  7. Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. pp. 279–280. ISBN   9780195106916.
  8. Levin, Harold L. (2010). The earth through time (9th ed.). Hoboken, N.J.: J. Wiley. p. 62. ISBN   978-0470387740.
  9. Klein & Hurlbut 1993, p. 496-497.
  10. 1 2 Nesse 2000, p. 285.
  11. 1 2 3 4 Wikisource-logo.svg One or more of the preceding sentences incorporates text from a publication now in the public domain :  Spencer, Leonard James (1911). "Amphibole". In Chisholm, Hugh (ed.). Encyclopædia Britannica . 1 (11th ed.). Cambridge University Press. pp. 883–884.
  12. US Geological Survey, Asbestos, accessed 20 July 2015.
  13. Nesse 2000, p. 242.
  14. "Health Effects of Asbestos". Agency for Toxic Substances and Disease Registry. Centers for Disease Control. Retrieved 6 November 2020.
  15. Nesse 2000, p. 286.
  16. Klein & Hurlbut 1993, pp. 495-496.
  17. Nesse 2000, pp. 287-289.
  18. "Pargasite" (PDF). Handbook of Mineralogy (pdf). Mineralogical Society of America . Retrieved 2012-12-17.

Related Research Articles

Mineral Element or chemical compound that is normally crystalline formed as a result of geological processes

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.

Hornblende Complex inosilicate series of minerals

Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole.

Actinolite

Actinolite is an amphibole silicate mineral with the chemical formula Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2.

Amphibolite A metamorphic rock containing mainly amphibole and plagioclase

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase.

Augite

Augite is a common rock-forming pyroxene mineral with formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.

Tremolite Amphibole, double chain inosilicate mineral

Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades to dark green with increasing iron content. It has a hardness on Mohs scale of 5 to 6. Nephrite, one of the two minerals of the gemstone jade, is a green variety of tremolite.

Silicate mineral Rock-forming minerals with predominantly silicate anions

Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.

Glaucophane

Glaucophane is the name of a mineral and a mineral group belonging to the sodic amphibole supergroup of the double chain inosilicates, with the chemical formula ☐Na2(Mg3Al2)Si8O22(OH)2.

Riebeckite Sodium-rich member of the amphibole group of silicate minerals

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na2(Fe2+3Fe3+2)Si8O22(OH)2. It forms a solid solution series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven, splintery. It is often translucent to nearly opaque.

Cummingtonite Mineral discovered and named after its place of discovery, Cummington, Massachusetts

Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe2+
)
2
(Mg,Fe2+
)
5
Si
8
O
22
(OH)
2
, magnesium iron silicate hydroxide.

Anthophyllite

Anthophyllite is an amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are classed as asbestos. The name is derived from the Latin word anthophyllum, meaning clove, an allusion to the most common color of the mineral.

Talc carbonates are a suite of rock and mineral compositions found in metamorphosed ultramafic rocks.

Tschermakite Amphibole, double chain inosilicate mineral

The endmember hornblende tschermakite (☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2) is a calcium rich monoclinic amphibole mineral. It is frequently synthesized along with its ternary solid solution series members tremolite and cummingtonite so that the thermodynamic properties of its assemblage can be applied to solving other solid solution series from a variety of amphibole minerals.

Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

Gedrite

Gedrite is a crystal belonging to the orthorhombic ferromagnesian subgroup of the amphibole supergroup of the double chain inosilicate minerals with the ideal chemical formula Mg
2
(Mg
3
Al
2
)(Si
6
Al
2
)O
22
(OH)
2
.

This list gives an overview of the classification of minerals (silicates) and includes mostly International Mineralogical Association (IMA) recognized minerals and its groupings. This list complements the List of minerals recognized by the International Mineralogical Association series of articles and List of minerals. Rocks, ores, mineral mixtures, non-IMA approved minerals and non-named minerals are mostly excluded.

Jimthompsonite

Jimthompsonite is a magnesium iron silicate mineral with chemical formula (Mg,Fe2+)
5
Si
6
O
16
(OH)
2
. It is a triple chain silicate (or inosilicate) along with clinojimthompsonite and chesterite. They were described in 1977 by Burham and Veblen. They attracted great mineralogical attention because they were the first examples of new chain silicate structures among a large group known as biopyriboles whose name is derived from the words biotite, pyroxene, and amphiboles.

Ferro-actinolite is the ferrous iron-rich endmember of the actinolite-tremolite continuous solid solution series of the double chain calcareous amphibole group of inosilicate minerals. All the series members belong to the monoclinic crystal system.

Coupled substitution Geological process by which two elements simultaneously substitute into a crystal

Coupled substitution is the geological process by which two elements simultaneous substitute into a crystal in order to maintain overall electrical neutrality and keep the charge constant. In forming a solid solution series, ionic size is more important than ionic charge, as this can be compensated for elsewhere in the structure.

References