This document provides information about amphibole group minerals. It discusses that amphiboles are double chain silicates that share properties with pyroxenes. Amphiboles have a Si:O ratio of 4:11, contain essential hydroxyl groups, and form prismatic or needle-like crystals. Several monoclinic and orthorhombic amphibole group minerals are described in detail, including their chemical formulas, crystal structures, typical occurrences, and physical properties. Prominent amphibole deposits in India are also listed.

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 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.

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

5.  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.

6. 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

7. 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

10. 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

11. 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

12. MONOCLINIC AMPHIBOLES

13. 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

14. 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.

15. 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.

16. 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

18. 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

19. 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

21. 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

22. 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 =105.75o ; Z=2
 Streak: White
 Luster: vitreous
 Diaphaneity: Translucent

24. 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.

25. ACTINOLITE SERIES
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.

27. 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.

28.  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.

30. 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.

32. 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