How Double-Chain
Silicates Break
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Remember that the double-chain silicates are two interlinked rows of bonded tetrahedra. The resulting structure is long and thin, as can be seen in this top view of a model of a double-chain silicate... |
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The double-chain is electrically charged and so must bond with a set of cations to become electrically neutral. |
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The ideal formula for a double-chain of silica tetrahedra is Si4O11-6. |
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These cations bond above and below the chain, and do more than just balance the charge... |
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The cations also help to bond one chain to the next... |
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The figure to the right represents a slice through a double-chain silicate, with the slice directly across the chains. |
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Each of the blue boxes represents a double chain of tetrahedra. (You can see the tetrahedra in the middle chains.) The white and green circles represent the cations that glue one chain to its neighbor. | |
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Fewer shared cations bond the chains together along the broad sides of chains compared to the narrow sides. Therefore these broad surfaces are bonded less strongly. It is along these weakly bonded surfaces, shown in red dashed lines, that the mineral will most likely break. |
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Simplifying the picture somewhat, you can see that the double-chain silicate can break into a set of fragments with potentially regular shape. |
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Looking at it in 3 dimensions, these potential breakages run along the chains. The chains themselves don't break as easily because the bonds between the silica tetrahedra are very strong compared to the bonds gluing one chain to the next. |
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The group of minerals called amphiboles are the most common double-chain silicates. Note how the shape of the amphibole crystals is similar to what we would have predicted based on its atomic architecture -- long, thin crystals that could be called fibers. |
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