1. SILICATE STRUCTURE AND CLASSIFICATION of
silicates
GOVT HOLKAR SCIENCE COLLEGE
GUIDED BY PRESENTED BY
Dr. NARENDRA JOSHI SHREYA BOBDE
Ms. TANISHKA SONI CLASS-M.Sc. I-
SEMESTER
2. CONTENTS
1. INTRODUCTION
2. THE SIO4 TETRAHEDRON
3. CLASSIFICATION OF SILICATES
4. NESOSILICATE
5. SOROSILICATE
6. INOSILICATES
3. a. SINGLE CHAIN
b. DOUBLE CHAIN
7. PHYLLOSILICATE
8. TECTOSILICATE
9. GENERAL FORMULA OF SILICATES
10. REFERENCES
4. INTRODUCTION
• Silicate makes up 90% of the earth’s crust.
• Of every 100 atoms in the crust of the earth, more
than 46 are oxygen, over 27 are silicon and 7 to 8
are aluminum.
• The rock forming silicate mineral include olivine,
garnet, pyroxenes ,amphibole, mica, feldspar and
quartz.
5. THE SIO4 TETRAHEDRON
• The structure of silicates consist of four O2-
at the
apices of regular tetrahedron surrounding and
coordinated by one Si4+
at the center.
• The bond between silicon and oxygen ions is
estimated by use of Pauling’s electron negativity
concept as 50% ionic and 50% covalent bond.
• Although electron sharing is present in the Si-O
bond, the total bonding energy of Si4+
is still
distributed equally among its four closest oxygen
neighbours .
6. • Strength of any single Si-O bond is equal to just
one half the total bonding energy available in the
oxygen ion.
• Each O2-
has therefore the potentiality of bonding
to another silicon ion and entering into another
tetrahedral grouping, uniting tetrahedral groups
through shared oxygen. Such linking of tetrahedra
is referred as polymerization.
• The capacity of polymerization is the origin of
great variety of silicate structures.
8. Geometry Of SiO4 Tetrahedron
• The shape of silicate structure is defined by Si-O
bond length and the O-Si-O bond angle.
• The mean Si-O bond length is 1.62 Å.
• When [SiO4
] tetrahedron are linked in a structure,
bond length between the silicon atom and the
bridging oxygen atom are on average longer by
about 0.025Å , compared with the Si-O bond
length of non-bridging oxygen.
9. • When tetrahedra are corner linked, tht Obr-Si-Obr
bond angle defines the orientation of tetrahedra
relative to one another.
• This bond angle can vary between 120o
to 180o
.
Depending upon temperature , pressure and local
structural environment.
10. CLASSIFICATION OF SILICATE
STRUCTURE
Class Arrangement of
SiO4 tetrahedra
Unit
composition
Mineral
example
Nesosilicate (SiO4)4-
Olivine , garnet
, zircon
Sorosilicate (Si2O7)6-
Epidote
Cyclosilicate (Si6O18)12-
Tourmaline ,
beryl
11. Class Arrangement of
SiO4 tetrahedra
Unit
composition
Mineral example
Inosilicate (Single
chain silicate)
(Si2O6)4-
Pyroxene ,
wollastonite
Inosilicate
(Double chain
silicate)
(Si4O11)6-
Amphibole
Phyllosilicate (Si2O5)2-
Mica , clay
minerals ,
serpentine ,
chlorite
Tectosilicate (SiO2) Feldspar, quartz
12. NESOSILICATE
• Nesosilicates are also known as Orthosilicates or
Island silicates.
• These are independent or isolated [SiO4]
tetrahedra and bound to each other only by ionic
bonds from interstitial cations.
• Their structure depends chiefly on the size and
charge of interstitial cations .
• Silicon to oxygen ratio in the chemical formula is
1:4.
13. • Atomic packing is generally dense, causing the
mineral of this group to have high specific gravity
and hardness.
• The crystal habit of nesosilicates is generally
equidimensional and cleavage directions are
absent.
• The amount of Al substituion in nesosilicates is
very low.
• E.g. olivine, garnet, zircon , kyanite.
14. Olivine (Mg,Fe)2SiO4
• Cation positions-
1. The octahederally sites
coordinated are known as
M1 and M2.
2. M1 is distorted and M2 is
somewhat more regular.
15. SOROSILICATE
• Sorosilicates are also known as pyrosilicates,
double island silicates or disilicates.
• These are characterized by isolated, double
tetrahedral, groups formed by two SiO4 tetrahedra
sharing a single optical oxygen.
• The resulting ratio of Si:O is 2:7.
• Chemical formula is [Si2O7]-6
.
• More than 70 minerals are known to this group but
most of them are rare.
• E.g. – Epidote group.
16. Epidote A2B3(SiO4)(Si2O7)O(OH)
• Contain both SiO4 as well as
Si2O7 group.
• Chains of AlO6 and AlO4
(OH) 2 runs parallel to b axis.
• A atoms are 8 fold
coordination between main
chains.
• B is generally Al, generally
substitute by ferric ion.
17. CYCLOSILICATE
• Cyclosilicates are also known as ring structures or
metasilicates.
• The cyclosilicate contain rings of linked SiO4
tetrahedra.
• The resulting ratio of Si:O ids 1:3.
• The simplest is the Si3O9 ring.
• Cyclic configuration of this kind may exist as-
18. Each of three tetrahedra shares an
oxygen atom. E.g. benitotite
BaTiSi3O9
Each of four tetrahedra shares an
oxygen atom. E.g. axinite
(Ca,Fe,Mn)3Al2BO3Si4O12OH
Each of six tetrahedra shares an
oxygen atom. E.g. beryl
Be3Al2Si6O18
19. INOSILICATE
• These are also known as chain silicates.
• Here SiO4 tetrahedra are joined together to form
chains of indefinite extent.
• These are of two types:-
(a) Single chain structure - Si:O ratio is 1:3.
Characterised by pyroxenes.
(b) Double chain structure - Si:O ratio is 4:11.
Characterised by amphiboles.
20. SINGLE CHAIN structures
• The SiO4 tetrahedra form linear single chains
with two bridging oxygen atoms per tetrahedron.
• In pyroxenes the periodicity is 2, i.e. the chain
repeats after every two tetrahedra.
• In pyroxenoids , longer chain periodicity occurs
such as in wollastonite (CaSiO3), where the
periodicity is 3.
• The Si:O ratio is 1:3 with general formula
(SiO3)n
2n-
.
• The chain is cross linked by cations, generally in
21. • Elongated in ‘c’-crystallographic direction.
• Pyroxenes are anhydrous minerals.
22. Pyroxene (XYSi2O6)
Cation positions-
• M1 sites lies between
apices of opposing tetrahedra.
Sites are smaller.
Almost regular tetrahedra.
• M2 sites are larger.
More distorted and may be
octahedra when containing
smaller cations or 8 fold sites
when occupied by larger
cations.
23. DOUBLE CHAIN structure
• These are also known as the ‘Band Structures’.
• The alternate tetrahedra are arranged in two
parallel ways.
• These chains are indefinite in extension .
• The Si:O ratio is 4:11 with general formula
(Si4O11)n
6n-
.
• Elongated usually in ‘c’-crystallographic direction.
• Amphiboles are hydrous minerals.
• Chains are not straight.
24. • Half of the tetrahedra have two non-bridging and
two bridging oxygens.
• The other half have one non-bridging and three
bridging oxygen.
• Amphibole formula - W0-1X2Y5Z8O22(OH, F) 2
25. Cation positions-
• Sites between tetrahedral
bases of adjacent chains
are termed as
M4.Coordination of M4 is
8 when occupied by larger
cations but reduces to 6
when occupied by smaller
cations.
• The smaller sites between
the opposed tetrahedral
apices are M1, M2 , M3
sites and are octahedral.
• A site has 10- to 12-
coordination with oxygen
and (OH).
26. PHYLlOSILICATE
• Phyllosilicates are also known as sheet structures.
• A sheet structure is formed when the SiO4
tetrahedra are linked by three of their corners.
• Extend indifinitely in a two dimensional network
or “sheet”.
• The ratio of Si:O is 4:10 with general formula
(Si2O5)n
2n-
.
• Most of its member have flaky or platy habit.
• They are generally soft , have low specific gravity
and show even elasticity of the cleavage lamellae.
27. • All these chracteristics are arises from the
dominance in the structure of the infinitely
extended sheet of SiO4 tetrahedra.
• Most of the minerals of this class are hydroxyl
bearing.
• E.g. mica group.
• (OH) group located in the center.
• External ions coordinated to
2 oxygens and 1 (OH).
• E.g. lizardite.
28. • The cations in the octahedral sheet may be divalent
or trivalent.
• On the basis of chemistry and geometry of the
octahedral sheets, the phyllosilicates are divided
into two major groups:- trioctahedral and
dioctahedral.
• A sheet in which each oxygen or (OH) group is
surrounded by three cations, is known as
trioctahedral structure.
• A sheet in which each oxygen or (OH) group is
surrounded by two cations, is known as
dioctahedral.
29. TECTOSILICATE
• It is also known as framework structure.
• When each of the four oxygen atoms of each
tetrahedron is shared by another tetrahedron, it
results in the formation of tectosilicates.
• Here every SiO4 tetrahedron shares all its corners
with other tetrahedra giving a three dimensional
network.
• The ratio of Si:O is 1:2.
• Here the bond is stable and strong.
• The framework is electrically neutral.
30. • There are at least nine different ways in which
such a frame work can be built.
• These modes of geometrical arrangement
corresponds to nine known polymorphs of SiO2,
one of which is synthetic. These are:-
1. Stishovite
2. Coesite
3. Low (α) quartz
4. High (β) quartz
5. Keatite(synthetic)
6. Low (α) tridymite
7. High (β) tridymite
31. 8. Low (α) cristobalite
9. High (β) cristobalite
• E.g. feldspar group.
32. feldspar
• It consisit of continous
negatively charged ion.
• Three dimensional
framework made up of
corner sharing SiO4 and
AlO4 tetrahedron and
positively charged cations.
• Cations occupy relatively
large interstices within the
framework.
33. General formula of silicates
• General structural formula for silicates as follows
XmYn(ZpOq)Wr
• X represents 8 to 12 fold coordination site for
large catios like K+
, Rb+
, Ba+2
, Na+
and Ca+2
.
• Y represents medium sized, two or four valent ions
in 6 coordination.
• Z represents small, highly charged ions in
tetrahedral coordination.
34. • O is oxygen.
• W represents additional anionic group such as
(OH)-
or anions such as Cl-
or F-
.
• The ratio p:q depends on the degree of
polymerization of the silica (or alumina)
tetrahedron or the silicate structural type.
• m, n and r depends on the need of electrical
neutrality.
35. references
• Andrew Putnis, Introduction to Mineral Science,
Cambridge University Press, Page 141 – 182.
• Dexter Perkins, Mineralogy, Pearson New International
Edition, Third Edition , Page 133 – 166.
• H.H. Read , Rutley’s Element Of Mineralogy, Surjeet
Publication, Twenty-Fifth Edition, Page 139-145.
• William D. Nesse, Introduction to Mineralogy, Oxford
University Press, Page 183- 325
• Websites-
https://www.britannica.com – Accessed on 27
september 2019
36. https://www.wou.edu/las/physci/ch412/silicate.htm
Accessed on 27 september 2019
https://opentextbc.ca/geology/chapter/2-4-silicate-
minerals/ - Accessed on 30 september 2019
http://butane.chem.uiuc.edu/pshapley/Environmental
/L27/1.html - Accessed on 1 october 2019
