AMPHIBOLE GROUP MINERALS

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-coordination

And 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’ 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.
Pyroxenes:

Crystals squat prisms

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
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

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

The 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)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.
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.
Grunerite Fe7 Si8 O22 (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.
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 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)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

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.