1. STRUCTURE OF THE SILICATES
1.Approx. 90% of the mineral content
of the earth’s crust is of silicates
where Si-O bonding, coupled with
different cations and anions, formed
different minerals.
2.The fundamental unit on which the
structures of all silicates are based
consists of four O2- apices of a
regular tetrahedron surrounding and
coordinated by one Si4+ at its centre.

2. STRUCTURE OF THE SILICATES
3. The Si-O bond is partly ionic
(generated due to attraction of opp.
charged ions) and partly covalent.
This tetrahedral groups can be linked
with adjacent tetrahedral groupings
through sharing of one or all four
oxygen atoms (Polymerization).
4. The mean Si-O bond length is 1.62Å.
Other cations near the oxygen atom
of a Si-O bond also attract the
oxygen and tend to lengthen the Si-0
bond( greater co-ord no; longer bond
length). The strength of the Si-O
bond limits the range of the bond
lengths from 1.60Å to 1.64Å

3. STRUCTURE OF THE
SILICATES
The bond length between Si atoms and the bridging
The bond length between Si atoms and the bridging
oxygen atoms (OBR) are on average longer by about
0.025Å compared with the Si-O bond length to the O nb.
[especially when SiO4 are linked in structures.
The OBR-Si-OBR bond angle is also shows slightly
smaller value when bridging oxygens are not involved.
This suggests that that the Si atoms are displaced
from the centres of the tetrahedra (awayfrom O BR) coz
of repulsive force between two Si atoms.
The aforesaid sharing of oxygens gave rise to diverse
structural configurations for silicates with various Si-O
ratios.

4. When tetrahedra are corner-linked, the Si-O bond angle
defines the orientation of the tetrahedra relative to one
another. This bond angle can vary between 1200 and 1800
depending on the local structural environment as well as
temp. and pressure. The bond angle of a strain-free Si-O-
Si bond is near 1400.
When Al substitutes Si in a tetrahedron, the [AlO4]
tetrahedra is slightly larger than a [SiO4] tetrahedra coz.
Al-O bond (1.75Å) is larger than the Si-O bond. When
SiO4and AlO4 are linked in a structure, this size
difference is accommodated by a change in the T-O-T
bond angle (T= tetrahedral cation)

5. SiO44- complex ion
• A group of ions that is so tightly bound together
that they act like a single unit.
• Building block of silicate minerals
• 1 silicon ion + 4 oxygen ions
arranged in a triangular pyramid
• Electrical charge of -4

7. Silicate tetrahedron
SiO44-

8. Tetrahedron Viewing
Top point
base
View from the top, Flat base of tetrahedron Side view
looking down. facing you. Top point
Top point of of tetrahedron pointing
tetrahedron away from you.
facing you

9. Nesosilicates (island silicates)
SiO44- tetrahedron forms ionic bonds with cations such as Mg2+, Fe2+
Example: Olivine
Mg2SiO4 - Fosterite Fe2SiO4 - Fayalite (Mg, Fe)SiO4
Solid solution
Mg2+ or Fe2+

10. Olivine: nesosilicate structure (island silicate)
View from the side (“wall” of structure)
Why Mg OR Fe?
• Same size
• Same electrical
charge
Peridot

11. Corner Sharing Base to Base
side view
O2-
Si4+
3-D Side View
© S. Brachfeld 2003

12. Inosilicicates: Single Chains (Pyroxene)
SiO3 Chain forms ionic bonds with cations above the tip
and below the base
Boxed region when view from
How many Si
How many O
2 Si
6O
Yellow tetrahedron in front
Gray behind and to the side
Cation, forming ionic
Stand here bond with tetrahedron
look up the chain chain

13. Corner Sharing Tip to Tip
O2-
Si4+
tesy of Donna Whitney, University of Minnesota Dept. Geology
Face sharing

14. Assembling a Single Chain Silicate (Pyroxene)
Build xl in this direction
© S. Brachfeld 2003

15. Pyroxene- Where’s the cleavage?
breaking at the molecular scale
What human eyes
see at the macro scale
90˚ cleavage
© S. Brachfeld 2003

16. Double Chain Silicates (Amphibole)
PAIR of SiO4 chains that link by corner sharing in 2 directions

17. Amphibole formula is long: lots of space for small
and medium cations
Cations include Na+, K+, Ca2+, Mn2+,
Fe2+, Mg2+, Fe3+, Al3+, Ti4+
Stand here
look up the chain

18. 60˚-120˚ Cleavage in Double Chain Silicates (Amphibole)- (actually 56˚ - 124˚)
breaking at the molecular scale
What human eyes see
at the macro scale
120 ˚
© S. Brachfeld 2003

19. Nesosilicates: independent SiO4 tetrahedra
b
c
projection
Olivine (100) view blue = M1 yellow = M2

20. Inosilicates: single chains- pyroxenes
b
a sinβ
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

21. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5
[(Si,Al)8O22] (OH)2
M1-M3 are small sites
M4 is larger (Ca)
A-site is really big
Variety of sites →
great chemical range
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

22. Stishovite
Tectosilicates
Coesite
Low Quartz β- quartz
α- quartz Liquid
Cristobalite
Tridymite
001 Projection Crystal Class 32