06.Periodicity of Behavior;
       Sodium Through Argon		   Previous PageNext Page
       Framework of the Planets: Silicates V

All these minerals cleave easily along the chain direction, but the covalent Si-O bonds within a chain are not easily broken. This is why asbestos (an amphibole) is fibrous and stringy.

Silicate tetrahedra also can be linked into endless sheets, with three of the four oxygen atoms shared, and only one O atom per Si left with a negative charge (bottom right).

This negative O is fully owned by one Si, while the other three are shared; thus the overall ratio of O to Si is 1+ 1/2 + 1/2 + 1/2 = 21/2 to one. The silicate sheet has the composition SiO2- or Si2O52-.

Even fewer metal ions are required to balance the negative charges than in olivines or pyroxenes, so micas and clays with sheet structures are lighter yet. They are believed to be present only in is the crust of the Earth.

The familiar flaking of mica arises because it i easy to separate silicate sheets, but much harder to break bonds within the sheets. (Recall the similar behavior of graphite.)

The most common materials in the crust of the Earth are the framework silicates: quartz and feldspars.

 

In these, all four corners of each silicate tetrahedron are linked in a three-dimensional framework. In common hexagonal quartz, the tetrahedra are linked in a six-fold helix, a spiral staircase with six silicate steps per turn.

These helices then are packed parallel to one another in the quartz crystal and connected by sharing oxygen atoms in the silicate tetrahedra. Hence quartz is held together completely by covalent Si-O bonds, and is a hard mineral.

There are left-handed and right-handed quartz crystals, depending on the direction or "handedness" of the helices.

If quartz is heated above its melting point of 1610°C and then cooled rapidly, the silicate chains do not have time to return to a perfectly crystalline array, so they harden into a disordered silicate glass instead.

Glass really is a liquid, although an extremely viscous one. Given enough time, glass will flow, as the thickening of the bottom of panes of glass in very old European buildings demonstrates.

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