AMPHIBOLE GROUP MINERALS

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 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-coordinationAnd Z for Si and Al in four coordination

The main physical and optical difference between the amphiboles and pyroxenes.Amphiboles:Crystals elongate prisms, often acicular or bladed.Basal sections are six sided and ‘diamond’ shapedTwo prismatic cleavages meet at 124 degree in the

basal sectionsUsually pleochroicMaximum extinction angle (measured to cleavage) on

an (010) section= ~20, biaxial negative(rarely positive) with large 2V (>70)

Exsolution lamallae very rare or not presentTwins common, with no re-entrant angle.

Pyroxenes:Crystals squat prismsBasal sections are eight-sided and square

shapedTwo prismatic cleavages meet at 88 degree in

the basal sectionNon-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

Monoclinic amphibolesCummingtonite 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

Orthorhombic amphibolesAnthophyllite (Mg,Fe)7 Si 8 O22 (OH)2

Gederite (Mg,Fe, Al)7 (Al, Si) 8 O22 (OH)2

Calcic amphiboles are monoclinicCa- free amphiboles are orthorhombicThe name hornblende is used by

petrologists to refer to the common black amphiboles found in many igneous and metamorphic rocks.

Anthophyllite (Mg,Fe)7 Si 8 O22 (OH)2Name 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.

cummingtonite (Mg, Fe)7 Si8 O22 (OH)2Named 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.

Grunerite Fe7 Si8 O22 (OH)2Named 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.

Tremolite Ca2 Mg5Si8 O22 (OH)2

Named after Val Tremola, Switzerland,. Where it was first found.

Tremolite is typically prismaticMg 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 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.

Hornblende (K,Na) 0-1 (Ca, Na, Fe, Mg)2 (Mg, Fe, Al)5 (Si, Al)8 O22 (OH)2Name 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 commonHornblende composition varies greatly

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.