|
Summary of minerals with AX4 coordination group structures |
. |
The number and type of linkage surfaces of close packed AX4
coordination groups |
. |
comprising the forgoing crystal structures depended on the percent
occupation of the |
. |
structure's tetrahedral voids with A atoms, the type of packing of the X
atoms, and |
. |
whether the occupied layers of the lattice
were successive or alternating. The range |
of linkage varied from AX4
groups that shared all six possible edges to groups that |
. |
shared no surfaces at all. Throughout this
progression these AX4 groups have |
. |
supplied all of the close
packed atoms necessary to build the lattice structure. |
. |
|
. |
The chemical composition of the crystal also depended on the percent
occupation |
of
the tetrahedral voids. A close examination of the lattice structures
demonstrates |
that the ratio of tetrahedral interstices to close packed X atoms in a FCC
or HCP |
crystal structure is 2:1. This enables the chemical formula of the
mineral to be |
deduced a priori from a knowledge of the occupation rate. This clearly
establishes |
the structural basis for the chemical composition of these minerals. |
. |
|
|
|
|
%
occupation |
formula |
|
|
100 |
A2X |
|
|
75 |
A3X2 |
|
|
50 |
AX |
|
|
25 |
AX2 |
|
|
1/6
(16.7%) |
AX3 |
|
|
1/8
(12.5%) |
AX4 |
|
. |
|
|
|
Table 6 - Percent occupation versus formula for AX4
mineral
structures |
|
|
|
|
|
The following section illustrates the effect that the percent occupation of
the |
octahedral voids of crystal lattice structures has on the geometry and
chemical |
composition of minerals. |
|
Back to
mineral table |
Back to
Knowhere |
Home page |
 |
Page
49 - Structure matters - Tetrahedral summary |
 |