Classes of Silicate structures Flashcards
What are the 3 classes of silicate structures?
- ) discrete anions
- ) One dimensional chains
- ) Two Dimensional sheets
- ) Framework silicates
What are examples of discrete anions (silicate structure)
Zircon ZrSiO4 Olivine M2SiO4 (M= Mg (2+), Fe(2+)) Beryl (ring structure) Be3Al2Si6O18
What are one dimensional chains (silicate structure)
- the simplest chain
- involves the sharing of two O atoms per SiO4 unit
- each SiO4 has two terminal O atoms
- so the repeat unit of the chain is SiO3 (2-)
cations Na+ and Al3+ balance the charge
what cations balance the charge of one dimensional chains (silicate structure)
Na+ and Al3+
What happens when two chains link laterally into a ribbon, what is the most common repeating unit?
Si4O11 (6-)
what is the purpose of metal ions that bind to the polyionic ribbons together?
to form neutral sheaths
why are these polyionic ribbon sheeths occur in fibrous strands (as in the family of asbestos minerals)
due to the weak intermolecular forces
What are two dimensional sheets?
- if 3 of the O atoms of each SiO4 unit are shared then INFINITE SERIES are formed.
- only one O atom per SiO4 unit is terminal and carries a negative charge
- all the terminal O atoms point in one direction one the same side of the sheet
- silicate repeat unit: Si2O5 (2-)
what is the repeat unit of 2 dimensional sheets
Si2O5(2-)
what is the repeat unit of one dimensional chains?
SiO3 (2-)
what is the sheet formula of Talc
Mg3(Si2O5)2(OH)2
true or false; all the bonding interactions among the ions and atoms of talc occur in a single layer
true
how are the layers of talc sheets attracted to each other?
van der waals interactions (intermolecular attractions)
what is the significance of each Talc layer being held by van der waals forces with each other?
allows the layers to slip over the other easily and hence talc feels slippery
how are aluminosilicates formed?
if some Si atoms in silicates are replaced by Al atoms (plus an appropriate cation)
what is the most abundant aluminosilicate
feldspars eg. albite (NaAlSi3O8)
true or false, mica is also an aluminosilicate
true
how are Al ions and other ions arranged in micas
they lie between aluminosilicate double layers
what is theformula of muscovite mica
KAl2(AlSi3O10)(OH)2
why do mica sheets not slide over each other so readily as in talc?
there is an ionic component to the interactions between the double sheets
true or false; mica can still be cleaved into layers
true
how is clay produced?
by weathering of silicate minerals such as talc and micas
what structure do clay usually have?
sheet structures with the layers held together by ionic interactions and hydrogen bonds
what happens when water is added to clay?
it causes the layers to SWELL and different cations can be found between the layers
what is a typical clay material
montmorillonite
In framework silicates the SiO4 units share how many oxygen atoms
4 Oxygen atoms
- to give a 3D framework silicate eg. silica (SiO2)
what does silica occur naturally as
SiO2 occurs naturally as quartz
what does granite consist of?
micro crystals of feldspar, mica and quartz
What are zeolites?
- 3 dimensional aluminosilicates
- the negative charge on the aluminosilicate framework is balanced by G1 or 2 cations
- usually open, porous structure with tunnels and channels (4-20A in diameter)
- found in nature but can also be made synthetically
- useful catalysts and absorbants
structure of zeolite
porous, open, tunnels and channels (4-20 A in diameter)
what are zeolites useful for?
catalysts and absorbants
