Amphibole group of minerals

Amphibole group of minerals

1. AMPHIBOLE GROUP MINERALS By- Romit and Prasant

2. Introduction Amphiboles are double chain silicates sharing many physical and chemical properties with pyroxenes. Inosilicates  are chain silicates which have interlocking chains of silicate tetrahedra with either SiO 3 , 1:3 ratio, for single chains or Si 4 O 11 , 4:11 ratio, for double chains. Amphibole  / is the name of an important group of generally dark- colored ,  inosilicate  minerals, forming prism or needlelike crystals Inosilicates  are chain silicates which have interlocking chains of silicate tetrahedra with either SiO 3 , 1:3 ratio, for single chains or Si 4 O 11 , 4:11 ratio, for double chains.

3. Amphiboles are composed of double chain  SiO4 tetrahedra , linked at the vertices and generally containing ions of iron and/or magnesium in their structures.

4. The amphiboles are a 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 structure and the Si:O ratio is 4:11, not 1:3 as in pyroxenes.

5. The main physical difference between the amphiboles and pyroxene group of minerals 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 Pyroxenes: Crystals squat prisms Basal sections are eight-sided and square shaped Two prismatic cleavages meet at 88 degree in the basal section

6. General formula: The general formula for all members of the amphibole group can be written as ( W,X,Y) 7 -8 ( Z 4 O 10 ) 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

7. Prominent occurance /mining in India Rajasthan- Ketri mines, Sikak district, Udaipur West Bengal- Purulia district Orissa- Sundargarh district Karnataka- Gulbarga district Tamil Nadu – Salem district

8. Orthorhombic amphiboles Anthophyllite ( Mg,Fe )7 Si 8 O22 (OH)2 Gederite ( Mg,Fe , Al)7 (Al, Si) 8 O22 (OH)2 Calcic amphiboles are monoclinic Ca- free amphiboles are orthorhombic

9. MONOCLINIC AMPHIBOLES

10. Monoclinic Amphiboles Cummingtonite series: 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

11. 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.

12. Occurance Cummingtonite is commonly found in metamorphosed magnesium-rich rocks and occurs in amphibolites. Usually it coexists with hornblende or  actinolite , magnesium clinochlore  chlorite Cummingtonite has also been found in some  felsic  volcanic rocks such as  dacites USA(Hampshire County, Massachusetts; Homestake gold mine, Lawrence County, South Dakota and La Paz County, Arizona, ); Scotland; South Africa and Sweden.

13. Physical properties: Color: Dark green,white , brown, gray, colorless to pale green in thin section Hardness : 5 – 6 Crystal habit : Columnar to fibrous and granular Crystal system : Monoclinic Streak : White Luster : vitreous Diaphaneity : transparent to translucent

14. 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 . Uses : Insulation

15. Physical properties: Color : Dark green or brown Hardness : 5 – 6 Crystal habit : Fibrous, bladed or columnar crystal Crystal system : Monoclinic a=9.6 ; b=18.3 ; c=5.3 ; β  =101.8 o ; Z=2 Streak : Colourless Luster : vitreous ,silky Diaphaneity : Translucent to opaque

16. Hornblende ( K,Na ) - 1 (Ca, Na, Fe, Mg) 2 (Mg, Fe, Al) 5 (Si, Al) 8 O 22 (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

17. Physical properties: Color : black or dark green Hardness : 5 – 6 Crystal habit : Prismatic, sometimes bladed,columnar or fibrous a=8.97 ; b=18.01 ; c=5.33 ; β  =105.75 o ; Z=2 Streak : White Luster : vitreous Diaphaneity : Translucent

18. Uses: It is crushed and used for highway construction and as railroad ballast. It is cut for use as dimension stone. The highest quality pieces are cut, polished and sold under the name "black granite" for use as building facing, floor tiles, countertops and other architectural uses. 

19. Tremolite Ca 2 Mg 5 Si 8 O 22 (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. ACTINOLITE SERIES

20. actinolite Ca2 ( 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.

21. Many end members have 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.

22. Glaucophane Na2 Mg3 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.

23. Riebeckite Na2(fe2+fe3+)si8O22(OH)2 Named after German explorer Emil Riebeck in 1888 Mohs hardness is 5.0-6.0,specific gravity 3.0-3.4,fracture is uneven, Diaphenity : translucent to nearly opaque Cleavage is perfect , two direction It form dark-blue elongated to fibrous crystal in highly alkali granites , syenites , schist and granite pregmatites Fibrous form of riebeckite is called crociodolite and is one of six recognised type of asbestos

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