Silicate Structures (Geology)

1. Silicate Structures
The building blocks of the common rock-
forming minerals
for
Geology (BSc.-II), Paper-202
Ms. Priyanka Shandilya
Assistant professor (Geology)
Govt. college Narnaul (Mahendergarh)
Haryana-123001
Prepared under the aegis of Directorate of Higher Education,
Haryana
Presented by

2. Mineral classes and the silicates
There are a total of 8 mineral categories (Dana
mineral classes)
Out of these most of the minerals are the
silicates which constitute ~92% of the Earth’s
crust
Because the SiO4 is not electronically neutral
(Si4+ + 4(O)2- = -4) it tends to combine with
other elements to make a neutral species

3. Silicates are compounds where Si and O are abundant and are
major mineral components of the earths crust and mantle
The basic unit for all silicates is the (SiO4)4-
tetrahedron.
The variety of silicate minerals is produced by the
(SiO4)4- tetrahedra linking to self-similar units sharing
one, two, three, or all four corner oxygens of the
tetrahedron.

4. (SiO4)4- tetrahedron
Three ways of drawing the silica tetrahedron:
a) At left, a ball & stick model, showing the silicon cation in
orange surrounded by 4 oxygen anions in blue;
b) At center, a space filling model;
c) At right, a geometric shorthand.
a b c
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5. Silicate Structures
6 types of Silicate Structures:
1. Nesosilicates
2. Sorosilicates
3. Cyclosilicates
4. Inosilicates
5. Phyllosilicates
6. Tectosilicates

6. Nesosilicates – (Orthosilicates)
• Isolated (SiO4)4- tetrahedra and bounded to one another via
ionic bonds with interstitial cations.
• Si:O=1:4
• Dense packing – high density.
• Independent tetrahedral – crystal habits are equidimensional
and lack pronounced cleavage.
• Al3+ substitution in T-site generallylow.
• Many nesosilicates (but not all) have orthogonal
crystallographic systems.

7. Olivine
www.geo.umass.edu

8. Olivine
Depiction of a single silicate
tetrahedron.
A picture of olivine (the green
crystals), an example of a silicate
structure composed of isolated
tetrahedrons.

9. Sorosilicates – (Disilicates)
• SiO4 tetrahedra in combination with Si2O7 units
• Si:O = 2:7
• These commonly form edge-sharing linked octahedral chains
• While bonds between chains and within sheets are quite
strong (large coordination spaces that accept large cations),
bonds between sheets are weak leading to the common
phenomenon of only 1 direction of perfect cleavage.
Epidote: Ca2(Al, Fe)Al2O(SiO4)(Si2O7)(OH)
wikipedia.org/wiki/
Sorosilicate

10. Cyclosilicates – (Ring silicates)
• 3, 4, 6, 8, 9, or 12 membered rings of Si4+ tetrahedra
(though 6-membered are the most common)
• Si:O = 1:3
• The high concentration of strongly linked bonds yield
relatively dense minerals that are quite hard
Beryl: Al2Be3Si6O18

11. Cyclosilicates – (Ring silicates)
www.adichemistry.com

12. (Si6O18)12-
Tetrahedral ring (6-fold)
Cyclosilicates or Ring silicates
(Si2O5)2-
Tetrahedral sheet (6-fold)
Phyllosilicates

13. Inosilicates - (chain silicates)
The most important two mineral groups
are the pyroxenes and the amphiboles.
Single Chain Double Chain

14. Pyroxenes
A schematic diagram of the
single chain silica structure.
Where two tetrahedra touch,
they share an oxygen ion.
Pyroxene is one of the
dominant minerals in this
sample of gabbro. It is the dark
mineral and can be hard to
recognize.
Si:O = 1:3

15. Amphiboles
A schematic diagram of the
double chain silicate
structure.
Hornblende is the dark
mineral in this rock.
Si:O = 4:11

16. Phyllosilicates – (Silicate Sheets)
(Si2O5)2-
Tetrahedral sheet (6-fold)
Many members have a platy
or flaky habit with one very
prominent cleavage.
Minerals are generally soft,
low specific gravity, may
even be flexible.
Most are hydroxyl bearing.

17. Phyllosilicates have a 2:5 ratio of tetrahedral
cations (such as Si) to oxygen
Some of the most common minerals in this
group are the micas (e.g. muscovite:
KAl2AlSi3O10(OH,F)2), but also many clay
minerals:
Kaolinite-serpentine: (Mg3, Al2)Si2O5(OH)4
Pyrophyllite: Al2Si4O10(OH)2

18. Sheets: Micas
An example of biotite.
An example of muscovite.
(Both biotite and muscovite
are micas, which are one kind
of sheet silicate.)
A schematic diagram of the sheet
silicate structure.

19. Now let’s talk about the silicate structures
which are completely linked and move
toward the less ordered structures that are
polymerized
Quartz and the Feldspars are some of the most
common framework silicates: ratio of Si to O
is 1:2
Albite:
Orthoclase:
Anorthite:
Na1+Al3+Si3O8
K1+Al3+Si3O8
Ca2+Al3+
2Si2O8

20. Tectosilicates – (Framework Silicates)
Infinite 3-dimensional network of (SiO4)4-
8 2 2 8
or (Si3Al)O 1- - (Si Al )O 2-
building blocks
All oxygen atoms are shared between
two SiO 4- tetrahedron.
4
Quartz is useful as a source of Si and for silica. It is used in
electronics as an oscillator, and is pizoelectric.

21. (SiO2)
Infinite tetrahedral network

22. The structure of feldspar is similar to that of the SiO2 polymorphs,
consisting of an infinite network of tetrahedra inter-connected via
bridging oxygen atoms.
In contrast to the SiO2 group, the tetrahedra may be AlO4 as well as SiO4.
Minerals are rendered electrically neutral as a result of being “stuffed”
with alkali or alkali-earth element cations in available voids.
(001)
(001)

23. • The white, blocky minerals in the
rock on the left are plagioclase
feldspar;
• the pink minerals in the rock on the
right (granite) are K-feldspar.
Feldspar Group
• An example of the
3-dimensional structure
formed by a framework
silicate

24. REVIEW
• 6 major silicate groups: Sorosilicates, Nesosilicates,
Cyclosilicates (Ring), Inosilicates (Chain), Phyllosilicates
(Sheet) and Tectosilicates (Framework), From Neso to Tecto,
these groups represent more polymerized structures
• There are multiple variations within each of these groups leading
to a variety of permutations for each; however, these are the
major building blocks of all silicate phases
• In addition to these are the non-silicates of which the
carbonates (CO3)2- dominate

25. slideplayer.com

26. References
http://www.geo.umass.edu/courses/geo311/systematic%
20min.pdf
https://www.visionlearning.com/en/library/Earth-
Science/6/The-Silicate-Minerals/140
https://www.adichemistry.com/inorganic/p-block/group-
14/silicates/silicates-1.html
https://slideplayer.com/slide/7384300/
https://fr.wikipedia.org/wiki/Sorosilicate

27. QUIZ
Q.1) Which one of the following is not a silicate
mineral?
a) Corundum
b) Quartz
c) Feldspar
d) Mica
Q.2) Which is not one of the silicates structures?
a) Neso Silicates
b) Soro Silicates
c) Inosilicates
d) Plicate

28. QUIZ
Q.3) What is the ratio of Si:O in nesosilicates?
a) 1:2
b) 2:5
c) 2:7
d) 1:4
Q.4) Which of the following minerals has the most
complex structure?
a) Epidote
b) Olivine
c) Quartz
d) Serpentine

29. Quiz
Q.5) Mention the name of one sheet silicate:
a) Boitite
b) Pyroxene
c) Ambhibole
d) Quartz
Q.6) Which of the silicate structures has Isolated (SiO4)4-
tetrahedra and bounded to one another via ionic bonds
with interstitial cations?
a) Sorosilicates
b) Nesosilicates
c) Inosilicates
d) Tectosilicates

30. Quiz
Fill in the blanks:
Q.7) __________is the basic unit of silicate minerals.
Q.8) Olivine crystallizes in ________ crystal system.
Q.9) Cleavage plane is _______ absent in quartz mineral.
Q.10) The Si:O ratio in tectosilicates is ______.

31. Contact details:
Ms. Priyanka Shandilya
pshandilya1210@gmail.com