Chemistry Reference
In-Depth Information
Rocks and minerals
1.1
WHAT IS A MINERAL?
A mineral is a
naturally occurring
homogeneous solid characterized by a
highly
ordered atomic structure
and characteristic
chemical composition
.
The International Mineralogical Association gives the following defini-
tion: “a mineral is an element or chemical compound that is normally crys-
talline and that has been formed as a result of geological processes” (Nickel,
1995). This definition excludes synthetic “minerals”.
1.1.1 An ordered atomic structure
Minerals are solids made of atoms arranged in a periodic and symmetric lat-
tice. Frankenheimer (1842) then Bravais (1848) have shown that there are
14 (and only 14) basic crystal lattice arrangements of atoms in three dimen-
sion types, referred to as the “Bravais lattice”. These lattices derive from
seven basic reticular systems:
triclinic, monoclinic, orthorhombic, tetrago-
nal, trigonal, hexagonal, cubic
. These systems are characterized by elements
of symmetry: centers, plans, reverse plans (symmetry with respect to a plane
and rotation), axes (order 2, 3, 4 or 6), reverse axes (rotation and symmetry
about center) (Figure 1.1).
The unit cell is the smallest crystal volume which has all the geometric
properties (symmetry, size), physical and chemical properties of crystal. It is
defined by the lengths of three vectors a, b, c and three angles
.
These three vectors form a basis, in which are identified in any plane or
any vector. In this basis, the equation of a plane is:
u x/y
α
,
β
,
γ
1
u, v and w are integers and are called
Miller indices
. The notation of a crys-
tal face or a plane is (h k l) (round brackets). By convention the notation
of a negative index (
+
v y/b
+
w z/c
=
u) is ( ¯). The vector normal to this plane has u, v, w
coordinates; so the notation of such direction is [uvw] (square brackets).
Hexagonal and trigonal systems use a set of coordinates and notations
slightly different: 3 axis at 120º (x y t) are used in the (x y) plane; the z axis is
−
