Abstract
THE various minerals belonging to the mica group are all characterized by the ease with which they cleave. The reason for this behaviour is found in their structure, which consists in all cases of a double sheet of linked silicon – oxygen tetrahedra of the hexagonal type (Si4O10)1. One in every four silicon ions is replaced by aluminium in most of the natural occurring micas (phlogopite, muscovite, biotite), thus giving the basic AlSi3O10 units; these are firmly bound by the incorporation of aluminium, magnesium and iron ions and hydroxyl groups. The double sheets are weakly linked by potassium ions, and it is along this plane that cleavage takes place. The most important micas conform to the idealized formulæ: Muscovite KAl2 (AlSi3O10)(OH)2 Phlogopite KMg3 (AlSi3O10)(OH)2, but a multitude of substitutions can take place; for example, the magnesium can to some extent be replaced by iron, which gives biotite: K(MgFe)3(AlSi3O10)(OH)2.
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POPPER, P. Transmission of Natural and Synthetic Mica in the Ultra-Violet. Nature 168, 1119–1120 (1951). https://doi.org/10.1038/1681119b0
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DOI: https://doi.org/10.1038/1681119b0
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