How Sheet Silicates Break
|
|
Sheet silicates are linked on all sides within a single plane. The resulting structure is a broad, flat, sheet, as can be seen in this top view of the following model of a sheet silicate bonding... |
|
|
|
||
 |
As with the previous example of the double chain silicate, the sheet of tetrahedra has a negative charge. ( The ideal formula for a double-chain of silica tetrahedra is Si2O5-2.) |
|
|
Consequently, a set of cations bonds with the sheet of tetrahedra making the overall crystal electrically neutral and bonding one sheet to the next. | |
|
The figure to the right represents a 3-D view of the crystal structure of a sheet silicate (in this case the mineral muscovite) |
|
|
Note that tetrahedra are arranged in sheets. The sheets are bonded together by two different means: by aluminum-bearing complex ions (yellow boxes) and by simple cations (light blue circles). | |
|
The aluminum-bearing complex ions bond strongly with the sheets of silica tetrahedra. |
|
|
The bonds between the simple cations and the sheets of silica tetrahedra are much weaker. It is along these weakly bonded surfaces, shown in red dashed lines, that the mineral will most likely break. | |
|
Looking at it in a simple 3-dimensional sketch, these potential breakages run between the sheets. The sheets themselves don't break as easily because the bonds between the silica tetrahedra are very strong compared to the bonds glueing one chain to the next. |
|
|
The group of minerals called micas are the most common sheet silicates. Note how the shape of the broken mica crystals is similar to what we would have predicted based on its atomic architecture -- broad, flat sheets. |
|
