Module Tag Min XIIb
Prof. Talat Ahmad
Prof. Naresh C. Pant
3. Structure and chemistry of amphibole
4. Crystallographic properties of amphibole
5. Physical properties of amphibole
6. Optical properties of amphibole
7. Occurrence of amphibole
8. Use of amphibole
To develop a basic understanding of chemical co-ordination leading to
formation of minerals with various structures.
To develop the ability to identify minerals in hand-specimen and thin section
To develop appreciation for the application of mineral science to everyday
life.
Inosilicates (Double chain)
According to the average atomic weight percent of eight most abundant elements in
the Earth oxygen (61.5%) is the most abundant element followed by silicon (21.2%).
This series continues with Al, Fe, Mg, Na and K. Silicates are most common
minerals in Earth’s crust (continental and oceanic crust) i.e. 92% (approximately).
We thus find that Earth’s crust can be dominantly considered as packing structure of
oxygen ion with interstitial Si, al, Fe, Mg, Ca, Na and K ions. In addition, the soil
from which we derive most of our food is dominantly made of silicates. A regular
tetrahedral is formed with Si4+ being in the centre and O2- at the apices. SiO4 is the
main unit in silicate structures; it may be an individual tetrahedral of tetrahedras
connected by sharing the apical oxygen and is the basis of classification of silicates.
This bonding to large extent also determines the physical and chemical properties of
minerals thus formed. Many silicate minerals also show substitution of Al for Si in
the tetrahedron which is accompanied by compensating replacement in cation
content to maintain charge neutrality.
Based on tetrahedral linkage amphiboles and pyroxenes are classified under
inosilicates, pyroxenes being single-chain inosilicates and amphiboles being double-
chains inosilicates. The two mineral groups thus share similar physical, chemical
and crystallographic properties. Both commonly have monoclinic crystals but both
also have orthorhombic crystals as well. Chain tetrahedral structure in both the
GEOLOGY
Paper: Crystallography and Mineralogy
Module: Inosilicates- Amphibole Group
groups extends along c-axis, and the c-dimension of unit cell in the two groups is 5.2
A, a cell dimension is also analogous but b-dimension is pyroxenes is almost half to
that of corresponding amphiboles. Framework cations are also same in both
amphiboles and pyroxenes but amphiboles contains water (OH) in their structures,
which is lacking in pyroxenes. The presence of water in amphiboles structures gives
them slightly lower specific gravity and refractive index than that of pyroxene.
Although the color, lusture, and hardness of analogous members are similar.
According to Bowen’s reaction series pyroxenes crystallize before amphiboles in
cooling igneous rocks, moreover under prograde metamorphic conditions
amphiboles commonly give way to pyroxenes whereas reverse is true for retrograde
metamorphism. Both of them have distinct crystal habits, pyroxenes tend to form
prisms while amphiboles mostly occurred as elongated crystals, often acicular.
3. Structure and chemistry
They are double-chain ino-silicates with Si:O ratio 4:11. Silica tetrahedral joins
by apical oxygen thus half of the tetrahedral sharing two oxygen (bridging
oxygen) and half-tetrahedral sharing 3 oxygen. The chain extends along C-axis
with C-axis repeat of 5.2 A. General formula to represent member of
amphiboles is W0-1X2Y5Z8O22 (OH,F)2, where W represents Na+, K+ in the A
site, X denotes Na+, Ca 2+, Mn2+, Fe 2+ Mg 2+ and Li+ in M4 site, Y denotes Mn2+,
Fe2+, Mg2+, Fe3+, Al3+, Cr3+, Ti4+ in M1, M2 and M3 sites and Z represents Si4+
and Al3+ in the tetrahedral site.
GEOLOGY
Figure: Double chain inosilicates, Tremolite.
The structure and behavior of amphiboles are similar to pyroxenes. The smaller sites
between opposite tetrahedral apices are known as M1, M2 and M3(M1 in
pyroxenes) and the sites between tetrahedral sites of adjacent chains is termed as
M4 site, M4 site is 8-fold coordination site when occupied by Ca 2+ but changes to 6
when occupied by smaller ion as Fe2+, Mg2+. Large A site may be filled by Na
GEOLOGY
Paper: Crystallography and Mineralogy
Module: Inosilicates- Amphibole Group
and/or Ca fully, may be partially filled or can also be vacant. A complete solid
solution occurs between Na and Ca species, and Mg, Fe and Mn series but Fe and al
and between Ti and other Y-type ions solid solution is limited. Although Al may
substitute for Si in tetrahedral site of double chain. F and O may also substitute Oh
ion in hydroxyl site. OH and F lies in the centre of hexagonal ring, at tetrahedral
apices level. Amphiboles are very diverse in composition and nature, hence are
classified as:
1) Ca rich amphiboles - High symmetry and occurs at wide temperature range
and composition.
2) Ca- poor amphiboles – Lower symmetry and collapse of chain on cooling.
3) Ca- free amphiboles – Orthorhombic structure at lower temperature changing
the relative position of chains along their length.
4) Protoamphibole – High temperature occurs as pure Mg end members at high
temperature.
The nomenclature of amphiboles is difficult due to wide range of chemical
substitution. At higher temperature solid solution over the whole range is possible in
amphiboles, while at lower temperature this substitution is controlled by size of M4
site and A site. An immiscibility occurs between some amphiboles of particular
chemical composition when,
1) M4 site is occupied by Na in one amphibole and Ca in other amphibole
2) M4 site is occupied by Ca-rich in one amphibole and Ca-poor in the other
3) A site is occupied in one amphibole and vacant in the other.
GEOLOGY
Mg
Al, Fe2+, Holmequistite
1) Sodic amphibole
When (Na+Ca)B is > 1.00 and NaB ranges from 0.50-1.50.
3) Calcic amphibole
Where (Na+Ca)B is > 1.00 and NaB is < 0.50, then the amphibole belongs
to calcic group usually but not always.
4) Magnesium-iron-magnese-lithium amphibole
(Na+Ca)B is < 1.00 and sum of (Mg, Fe, Mn, Li)B is > 1.00
Figure. General classification of amphiboles, excluding the Fe-Mg-Mn-Li
amphiboles.
GEOLOGY
GEOLOGY
5.2 Amphiboles in hand specimen
Amphiboles shows a wide range of composition and hence color varies from grey to
various shades of green and brown and beige. Cleavage is perfect along plane
{210}, intersecting at 56 t1nd 124 . Hardness varies from 51/2-6. Specific gravity
ranges from 2.85-3.2. They show vitreous luster and white to grey streak.
6. Optical properties
6.1 Amphiboles under plane polarized light
Amphiboles are mostly light colored with peak pleochroism, α = colorless, β = pale
yellow brown; γ = pale green, absorption formula γ > β ≥ α. They have columnar,
bladed or acicular habit mostly. Refractive index is high. They shows distinct
prismatic cleavage.
They are anisotropic in nature, having moderate to high birefringence. Amphiboles
show bright polarization color ranging from first order to mid third order. Extinction
angle is again a function of Fe content, being 21 in Mg rich to 12 in Fe rich
members of the series. Extinction is parallel to cleavage in {100} section but
symmetrical in basal sections. Orientation is α X = -9 to -12, β = γ, γ z = -21
to 12°. Biaxial indicatrix changing from positive to negative when grunerite
component exceeds 70%. 2Vα ranges from 7-90. Twinning may be simple contact
or repeated on {100} and twin lamellae are often narrow. They may also show
zoning with rim of hornblende, or may form as a reaction rim on pyroxenes.
GEOLOGY
Paper: Crystallography and Mineralogy
Module: Inosilicates- Amphibole Group
7. Occurrence of amphiboles
Anthophyllite is formed as a result of metamorphism of Mg-rich rocks as ultrabasic
igneous rocks and dolomitic shales. It is also found in cordierite-bearing gneisses
and schist. Cummingtonite occurs in regionally metamorphic rocks and in
amphibolites, coexisting with hornblende, actinolite, and Mg-rich commingtonite.
Cummingtonite sometimes also found as phenocrysts in some igneous rocks as
dacite. Tremolite is a characteristic mineral of greenschist facies of metamorphism.
It also occurs in metamorphosed dolomitic limestone and changes to diopside at
higher temperature. Hornblende is an amphibole with large compositional range
with variation in the ratio Ca/Na, Mg/Fe, Al/Fe, Al/Si and OH/F occurs both in
igneous and metamorphic rocks. It is found in medium grade metamorphic rocks
called as amphibolite and associated with plagioclase. It is commonly found in
syenites and diorites. Glaucophane is commonly found in metamorphic rocks as in
bluschisht metamorphic facies and in other schists, eclogites and marbles in
association with jadeite, lawsonite and aragonite. Riebeckite also found in igneous
rocks as granites, syenites, nepheline syenites and related pegmatites. Pyroxene
minerals Enstatie (Mg, Fe)SiO3 and Ferrosilite (Mg,Fe)SiO3 are used as minor gem
minerals. Pigeonites are also used as gem minerals.
8. Uses of amphiboles
Some varieties of amphiboles are used as gemstone and for ornamental purposes as
nephrite. Amphiboles are also used for asbestos. Crocidolite is used for production
of asbestos, constituting 4% of world production.
GEOLOGY
Q1. What causes the typical amphibole cleavage?
Q2. Compare the unit cell dimensions of amphiboles and pyroxenes and discuss the
effect of double chains on unit cell dimension?
Q3. What is the significance of A-site in amphibole structure?
Multiple Choice Questions-
(a) Almost half of the corresponding pyroxene
(b) Almost same as that of the corresponding pyroxene
(c) Almost double of the corresponding pyroxene
(d) None of the above
Ans: c
2. In the general formula of amphiboles as W0-1X2Y5Z8O22(OH,F)2, the vacancy
exists in teemolite in
Ans: d
3. This site in amphibole structure is 8-fold coordination when occupied by Ca2+ but
changes to 6-fold when occupied by Fe2+ or Mg2+
(a) M3
(b) M4
(c) M2
(d) M1
Ans: b
(a) Anthophyllite
(b) Actinolite
(c) Riebeckite
(d) Hornblende
Ans: a
Paper: Crystallography and Mineralogy
Module: Inosilicates- Amphibole Group
5. This amphibole is a mineral typically characteristic of greenschist facies
metamorphism of mafic rocks
(a) Tremolite
(b) Glaucophane
(c) Edenite
(d) Grunerite
Ans: a
Suggested Readings:
1. Klien, Cornelis and Hurlbut, Cornelius S., (1985). Manual of Mineralogy
(after James D. Dana), 20th Edn. John Wiley & Sons, New York. ISBN:
0471805807, 978-0471805809.
2. Putnis Andrew (1992), An Introduction to Mineral Sciences, 1st Edn.,
Cambridge University Press, UK. ISBN: 0521429471, 978-0521429474.
3. Leake B. E. et. al., (1997) Nomenclature of Amphiboles: Reports of
subcommittee on amphiboles of the international mineralogical association,
commission on new minerals and mineral names. The Canadian
Mineralogist Vol. 35, pp. 219-246.
Table of Content
1. Learning outcomes
3. Structure and chemistry