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The Movement of ions through rocks


I mentioned previously that ions can be pulled through materials such as glass by the application of a direct current. Like glass, most rocks are made from silica and are not crystalline. This is because silica is formed from the covalent bonding of silicon atoms with atoms of oxygen. In covalent bonding electrons are shared between two or more atoms and a fixed directional relationship between the atoms is required. Silica has a tetrahedron configuration.


Silicon has a valence of four and shares electrons with four oxygen atoms giving a total of eight electrons for each silicon atom. Oxygen has a valence of six and shares electrons with two silicon atoms giving it a total of eight electrons. One or more of the oxygen atoms in the silica arrangement can share electrons with silicon atoms in other silica units and this means that polymers can be formed.


In this way a double unit (pyrosilicate), a chain, a ring structure, or even a complex network can form. The spaces between chains or rings or sheets can be filled by cations such as sodium and magnesium and these ions determine the properties of the mineral or rock. For example, rings can be linked together in platelets or sheet structures - as in kaolinite.


The mineral olivine is an example of the configuration of single silica units with magnesium and iron cations held between them. As one would expect, olivine with its simple structure is present in amorphous mafic rocks such as basalt and gabbro.


When glass is  rrapidlycooled a network of silica forms. Modifiers such as sodium oxide can be added to molten glass so that the network formed is less complex.




























Glass can form naturally as obsidian when lava is quenched rapidly. Tektites are glass beads which are formed when molten silica is rapidly cooled in the atmosphere after an extremely violent volcanic event.


Aluminium atoms are also covalently bonded to oxygen atoms and form a range of equivalent structures to silica. In hydrothermal conditions aluminium oxide (alumina) can be hydrated to the hydroxide, gibbsite which is a component of bauxite.


Alumina and silica units can together form minerals such as andalusite.


Tension on all of these structures causes their chains to align in the plane of stress. At extreme strain conditions they can form fibres or leaves. One would expect micas to be formed in areas where there has been maximum strain. Indeed micas are to be found in areas parallel with some coastlines - and these will have been zones of maximum tension before continents split away from each other.


In previous chapters I suggested that a toffee bar being stretched apart as an analogy for the way continental crust stretches, necks and finally fails. Toffee with its chains of polysaccharides, randomly arranged in an amorphous structure, is very similar to many rocks. If a toffee bar is pulled apart, the chains of polysaccharides will align in the plane of the strain; creep between chains occurs, and the most strained area in the middle of the bar becomes translucent prior to its breaking apart. Temperature increase also assists this

process - warm toffee stretches much more easily than cold toffee.








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