Structural mineralogy 101

Structural
Mineralogy
By: Ahmed Essam

Crystal Chemistry
• What is Lattice?
• Lattice is a regular network of points which represents
atoms of elements usually major elements forming a
certain mineral.
• Lattice points or atoms are usually connected by lines
which represents different types of bonds between
atoms.
• Space lattice is a regular repetation of lattice points in
space in 3 dimensions at constant interval.
• Unit cell is the smallest building unit of crystal lattice
which usually a box-like in 3 dimensions.
• Lattice plane is a plane or a group of parallel planes
which intersect the lattice and divide it into layers.
2By: Ahmed Essam Ayad

Crystal Chemistry
By: Ahmed Essam Ayad 3

Crystal Chemistry

Crystal Chemistry
The usual choice
of unit cell face
s u r f a c e i s t h e
planes which are
densely packed
w i t h l a t t i c e
points.

Crystal Chemistry

Crystal Chemistry
• Unit cell Types:-
A. Primitive " Simple " cell ( P ): one atom at each corner.

Crystal Chemistry
B. Compound cell:
1- Body centred: one atom at each corner and one in
the center.
2- Face centred: one atom at each corner and one at
each end of crystallographic axes.
3- Side centred: one atom at each corner and one at
the ends of just single crystallographic axis " on two
opposite faces ".
I. a - side centred: at the ends of a-axis.
II. b - side centred: at the ends of b-axis.
III. c - side centred: at the ends of c-axis.

Crystal Chemistry

Coordination
• Coordination number (Kz ) : the number of atoms
or ions usually anions surrounding a central atom
or ion usually cation in a complex or lattice
structure.
• Examples:
• Kz of linear structure = 2 , it means 2 anions surrounded a
cation opposite to each other to form linear structure.
• Kz = 3 , forms triangular structure.
• Kz = 4 , forms tetrahedral structure as in SiO4 silicon
surrounded by 4 anion of oxygen to form tetrahedron.
• And so on;

Coordination

Coordination
• Coordination of Ions:
• Radius ratio Rx/Rz, where Rx is the radius of the cation,
and Rz is the radius of the surrounding anions.
• The anions are usually the larger ions, this results in
decreasing values of Rx/Rz as the size of the cation
decreases.
• In the following figure if the size of cation is small enough
to allow the 8 anions to touch each other as well as cation it
forms whats called 8-fold coordination or cubic
coordination.

Coordination
4 - fold coordination 3 - fold coordination
2 - fold coordination

coordination
• Table summarizes the cation to anion radius
ratios, Rx/Rz, for various coordination numbers
and gives the name of the coordination
polyhedron for each coordination number.

Coordination
Table gives the
ionic radius and
coordination of
the common
metal cations
coordinated
with oxygen.

Structural Classification
OF Minerals
• Metallic Bond: it is the sharing of many detached
electrons between many positive ions, where the
electrons act as a "glue" giving the substance a
definite structure.

OF Minerals
• Metallic Minerals:
• Homodesmic: Metals and their alloys.
• A1 : Cubic face centred.
• A2 : Cubic body centres.
• A3 : Hexagonal Face centred.
• ex : Gold, Silver,......etc.
• Heterodesmic: Most Sulphide Minerals.
• ex : Pyrite, Marcasite, Molybdenite,......etc.

OF Minerals

OF Minerals
• Covalent or Molecular Bond: is a chemical bond
that involves the sharing of electron pairs between
atoms.
• These electron pairs are known as shared pairs or bonding
pairs, and the stable balance of attractive and repulsive
forces between atoms, when they share electrons, is known
as covalent bonding.

OF Minerals
• Examples of Minerals with Covalent bond :-
• Diamond C:

OF Minerals
• Sphalerite ZnS:

OF Minerals
• Cuprite Cu2O:

OF Minerals
• Sulfur S: 8 atoms puckered ring.

OF Minerals
• Graphite C:

OF Minerals
• Ionic Bond: is the complete transfer of valence
electron(s) between atoms. It is a type of chemical
bond that generates two oppositely charged ions.
• In ionic bonds, the metal loses electrons to become a
positively charged cation, whereas the nonmetal accepts
those electrons to become a negatively charged anion.

OF Minerals
• Ionic Minerals:-
• Isodesmic:
• Simple: One cation such as Bonding in Halides ex.
( Halite, Periclase, Sylvite, Flourite ) also in minerals
like ( Uraninite, Rutile, Cassiterite ).
• Multiple: More than one cation. ex. Spinel Group.

OF Minerals

OF Minerals
• Anisodesmic: Bonding in Sulphates, Nitrates, Phosphates,
Carbonates ex. Calcite.

OF Minerals
• Mesodesmic: Bonding in silicates.

THANKS
• E-mail: geo.ahmedessam@gmail.com
35By: Ahmed Essam Ayad

Structural mineralogy 101

More Related Content

What's hot

Similar to Structural mineralogy 101

Recently uploaded

Structural mineralogy 101