STRUCTURE OF A 2:1 PHYLLOSILICATE MINERAL (MUSCOVITE - MICA)
Basic Muscovite structure: Red = Oxygen, Purple = Potassium, Grey= Silicon, and Brown = Aluminum. In the view at left, the hydrogen ions have been deleted and the structure expanded to view the ion locations and bonds. Note that each Si ion is surrounded by four O ions. (Don't be disturbed by the fact that there are no bonds between the Si and some of the O ions; this is a function of the program which produced the illustration.) It is a little more difficult to see, but each Al ion is surrounded by six O ions. In the vertical direction (mineralogically, the "c axis") the repeat structure goes from one K ion to the next and contains a layer of Al ions in octahedral coordination sandwiched between two layers of Si ions in tetrahedral coordination.
In reality, if we could view the atoms of the Muscovite mineral it would look something a little closer to the illustration at the right. The primary problem is that all ions are depicted the same size. In reality, the Si and Al ions would be much smaller and nearly hidden behind the O ions. The white dots on the lower O ions are the Hydrogen ions, which are more nearly in correct size relationship to the O ions. In this view it is a little easier to see that each Al ion is surrounded by six O ions, and that the Al ions share two O ions. While it is somewhat more difficult to visualize, the Si ions share only one O ion.
Now lets build our muscovite mineral, keeping our perspective perpendicular to the c-axis. This begins to provide a better concept of mineral morphology. Between the layers of K ions we see successive layers of oxygen, silicon, oxygen, aluminum, oxygen, silicon, and oxygen. This structure is refereed to as a "mycelle". The thickness of the mycelle being formed is about one nanometer. On the a- and b-axes the mycelle will usually extend 10 nanometers or more.
Finally, I will tip the mineral slightly so you can view it along the a- and b-axes. You see here that the K ions form a layer on the surface of the mycelle. These ions function to bind the mycelles together forming the muscovite mineral found in nature. If you look closely, you will see that each K ion is surrounded by six O ions. If we were to successively peel back the O layers, you would see that the two central layers are in a close-packed structure, but that the two outer layers are not. Since the Si ions can only share a single O ion, the O ions in these layers form into six member "rings" and K is just the correct size to fit into the empty space at the center of the ring. This depicted structure is still too small to be seen even with the help of a microscope. The mica you may have picked up while hiking along mountain trails will consist of many mycelles extending much farther on the a- and b-axes.
To provide a degree of perspective, a muscovite particle such as shown at left would be about .2 x .2 x .2 micrometers in size. In other words, it would be a fine clay size particle, still not large enough to see even with the help of a microscope.
BACK TO "BUILDING THE PHYLOSILICATES"
BACK TO SYLLABUS