Amphiboles

Amphiboles

• 1.
  AMPHIBOLE GROUP MINERALS
• 2.
  The amphiboles area group of hydroxylated chain silicates with some substitution of F and Cl for (OH). They include both orthorhombic and monoclinic members. The amphiboles contain essential (OH) group in the structureand the Si:O ratio is 4:11, not 1:3 as in pyroxenes. The general formula for all members of the amphibole group can be written as ( W,X,Y) 7-8 ( Z4 O10)2 (OH)2., In which the symbols W,X,Y,Z indicate elements having similar ionic radii and being capable of replacing each other in the structure. W stands for large cations Ca and Na sometimes K. X for smaller cations Mg and Fe2+ sometimes Mn Y for Fe3+,Ti, and Al in six-coordination And Z for Si and Al in four coordination
• 3.
  The main physicaland optical difference between the amphiboles and pyroxenes. Amphiboles: Crystals elongate prisms, often acicular or bladed. Basal sections are six sided and ‘diamond’ shaped Two prismatic cleavages meet at 124 degree in the basal sections Usually pleochroic Maximum extinction angle (measured to cleavage) on an (010) section= ~20, biaxial negative(rarely positive) with large 2V (>70) Exsolution lamallae very rare or not present Twins common, with no re-entrant angle.
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  Pyroxenes: Crystals squatprisms Basal sections are eight-sided and square shaped Two prismatic cleavages meet at 88 degree in the basal section Non-pleochroic except for the Na-Fe3+bearing varieties. Maximum extinction angle (measured to cleavage) on an (010) section= ~48, biaxial positive(rarely negative) with large 2V (50-60) Exsolution lamallae common in cpx and opx from basic igneous rocks. Twins rare with a re-entrant angle
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  Monoclinic amphiboles Cummingtoniteseries: cummingtonite (Mg, Fe) 7 Si 8 O 22 (OH) 2 Grunerite Fe 7 Si 8 O 22 (OH) 2 Actinolite series: Tremolite Ca 2 Mg 5 Si 8 O 22 (OH) 2 actinolite Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) 2 Hornblende (K,Na) 0-1 (Ca, Na, Fe, Mg) 2 (Mg, Fe, Al) 5 (Si, Al) 8 O 22 (OH) 2 Na- amphiboles: Glaucophane N a 2 Mg 3 Al 2 Si 8 O 22 (OH) 2 Riebeckite N a 2 ( Fe,Mg) 3 (Fe,Al) 2 Si 8 O 22 (OH) 2
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  Orthorhombic amphiboles Anthophyllite(Mg,Fe) 7 Si 8 O 22 (OH) 2 Gederite (Mg,Fe, Al) 7 (Al, Si) 8 O 22 (OH) 2 Calcic amphiboles are monoclinic Ca- free amphiboles are orthorhombic The name hornblende is used by petrologists to refer to the common black amphiboles found in many igneous and metamorphic rocks.
• 7.
  Anthophyllite (Mg,Fe)7 Si8 O22 (OH)2 Name from the Latin anthophyllum , meaning ‘clove leaf’ referringt to this mineral’s colour. In hand specimen it is characterized by it its clove- brown colour, usual prismatic habit, and prismatic cleavages. Anthophyllite is usually Mg- rich; iron rich compositions yield cummingtonite. Al and Na may be present in anthophyllite; if Al content is great enough, the amphibole is called gederite. Anthophyllite is found in low- grad Mg- rich metamorphic rocks where it may be associated with cordierite. Amosite is asbestiform variety of anthophyllite.
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  cummingtonite (Mg,Fe) 7 Si 8 O 22 (OH) 2 Named after Cummington, Massachusetts, its type locality. Forms prismatic, fibrous crystals,aggregates of radiating fibers or blades are common. Occurs in mafic or marly medium grade metamorphic rocks. Amosite is asbestiform amphibole similar to Fe- rich comingtonite. Commingtonite is closely related to the other ambhiboles and is polymorphic with the members of the anthophyllite series.
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  Grunerite Fe 7 Si 8 O 22 (OH) 2 Named after L.E Gruner, a 19 th century mineralogist, who first analyzed grunerite. Typically forms fibrous, bladed or columnar crystals often radiating. Found with Fe- rich minerals such as hematite, magnetite, hedenbergite, fayalite or garnet in metamorphosed iron rich sediments.
• 10.
  Tremolite Ca2 Mg5Si8 O22 (OH)2 Named after Val Tremola, Switzerland,. Where it was first found. Tremolite is typically prismatic Mg rich endmember of the calcic amphibole series. Complete solid solution between tremolite and Fe- actinolite. Intermediate compositions are simply termed as actinolite. Tremolite Is one of the first mineral to form when impure carbonate are metamorphosed.
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  actinolite Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) 2 Name from the Greek actis (ray), referring to its common habit of radiating needles. Actinolite typically form needles- either radiating or in parallelel aggregates, or columnar massees. Actinolite is characteristic of medium- grade metamorphosed mafic rocks. It is one of the minerals that gives greenschists their characteristic colour. Associated minerals typically include albite, epidote, chlorite and quartz.
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  Hornblende (K,Na)0-1 (Ca, Na, Fe, Mg)2 (Mg, Fe, Al)5 (Si, Al)8 O22 (OH)2 Name from the German horn (horn) and blenden (blind), referring to its luster and its lack of value. In hand specimen Habit, cleavages and dark colour usually serve to identify hornblende. Habit may be massive or prismatic and is sometimes bladed, columnar, or fibrous. Contact twins are common Hornblende composition varies greatly
• 13.
  Many end membershave names; some of the more commonly used ones are Edenite, ferro-edenite, pargasite, ferro- pargasite, tschermakite, ferro- tschermakite, tremolite, ferro actinolite, glaucophane, kaersutite. Besides compositional variations some hornblende varieties include F- or O2- substituting for OH, Or Fe 3+ substituting for Fe 2+ Hb is common in many kinds of igneous rocks covering a wide range of composition. Associated with plagioclase, quartz or pyroxene and/or olivine. Also found in metamorphosed mafic rocks, especially in amphibolite.
• 14.
  Glaucophane Na2Mg3 Al 2Si8 O22 (OH)2 name from Greek words meaning “to appear bluish” Acicular, asbestiform or fibrous habit characterizes glaucophane. Compositions intermediate between glaucophane and riebeckite are called Crossite. Glaucophane is a high pressure metamorphic mineral characteristic of blue schist facies. Other blue schist minerals include jadeite, lawsonite and aragonite.