AMPHIBOLE GROUP MINERALS

By- Romit and

Prasant

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 SiO3, 1:3 ratio, for single chains or Si4O11, 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 SiO3, 1:3 ratio, for single chains or Si4O11, 4:11 ratio, for double chains.

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

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.

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

General formula:

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

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

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

MONOCLINIC AMPHIBOLES

Monoclinic Amphiboles Cummingtonite series:cummingtonite (Mg, Fe)7 Si8 O22 (OH)2

Grunerite Fe7 Si8 O22 (OH)2

Actinolite series:

Tremolite Ca2 Mg5Si8 O22 (OH)2

actinolite Ca2 (Mg,Fe)5Si8 O22 (OH)2

• Hornblende (K,Na) 0-1 (Ca, Na, Fe, Mg)2

(Mg, Fe, Al)5 (Si, Al)8 O22 (OH)2

• Na- amphiboles:

Glaucophane Na2 Mg3 Al 2Si8 O22 (OH)2

Riebeckite Na2 ( Fe,Mg)3 (Fe,Al) 2Si8 O22 (OH)2

cummingtonite (Mg, Fe)7 Si8 O22 (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.

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; Homestakegold mine, Lawrence County, South Dakota and La Paz County, Arizona, ); Scotland; South Africa and Sweden.

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

Grunerite ( Fe7 Si8 O22 (OH)2) Named after L.E Gruner, a 19th 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

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 =101.8o ; Z=2

Streak: Colourless

Luster: vitreous ,silky

Diaphaneity: Translucent to opaque

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 colourusually 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

Physical properties:

Color: black or dark green

Hardness: 5 – 6

Crystal habit: Prismatic, sometimes bladed,columnaror fibrous

a=8.97 ; b=18.01 ; c=5.33 ; β =105.75o ; Z=2

Streak: White

Luster: vitreous

Diaphaneity: Translucent

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.

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.

ACTINOLITE SERIES

actinolite Ca2 (Mg,Fe)5Si8 O22 (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.

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.

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.

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