THE BRICKS WITH WHICH MINERALS ARE BUILT

KAOLINITE (1:1) MINERALS

2:1 MINERALS

CHLORITE MINERALS

DIOCTAHEDRAL/TRIOCTAHEDRAL MINERALS

SOURCE OF CHARGE

RETURN TO SYLLABUS

There are five polyhedra that are basic building blocks of crystals.

Of these only the tetrahedron and octahedron will be of interest to us for building the phyllosilicates. Lets look first at the basic tetrahedral unit. The Si - O combination has a radius ratio of 0.30, which means that the silicon ion fits nicely into a tetrahedral polyhedron. An exploded schematic diagram of this polydedron is shown at left. We could talk about electron orbitals and discuss just why this happens, but lets keep the discussion simple. Lets just say that the Silicon ion shares its charge equally between the four oxygen ions, leaving each oxygen with an excess charge of negative one. We now have the orthosilicate anion, which could at least theoretically be neutralized by four protons (hydrogen ions). This anion tends to react readily with alkali and alkali earth ions. The SiO₄^4- anion does have another option open to satisfy the charges, however.
 

It is also possible for an oxygen ion to bond with two Si ions, and thereby have its charge balanced. Theoretically this could happen by the three face oxygen ions, the two edge oxygen ions, or the single corner oxygen ion bonding with two silicon ions. There may be several reasons that the first two options are not possible, but in the simplest explaination either face sharing or edge sharing would bring the two highly electropositive silicon ions too close together. Thus, only a sharing of the corner oxygen ion is a viable option. This can and does occur. If each of the four oxygen ions bond with two silicon ions the result is a QUARTZcrystal. In the phyllosilicates only one plane of oxygen ions bond with two silicon ions as indicated at right. This bonding is extended in two directions to form a sheet of silicon tetrahedrons. Lets for the moment leave this sheet of tetrahedral units with unbalanced charges on the apical O ions.

The second basic building block of the phyllosilicates is an aluminum octahedral unit. The aluminum/oxygen radius ratio is 0.41, which we see from the above table falls right at the maximum ratio for tetrahedral coordination and minimum ratio for octahedral coordination. Depending on conditions, aluminum can coordinate with either four or six oxygen ions. As it turns out, however, within the phyllosilicate mineral structure the aluminum ion is "more comfortable" in an octahedral coordination. An exploded view of the aluminum octahedral unit is demonstrated at right. Using the same simplified approach that we used to explain the Si tetrahedral unit, aluminum might be said to share +0.5 of its charge with each of the surrounding oxygen ions, leaving each oxygen ion with a negative 1.5 charge.

This excess negative charge on the oxygen ions needs to be balanced and the charge can at least be partially balance if each oxygen ion is bonded with two aluminum ions. Once again, this could theoretically happen by the three face oxygen ions, the two edge oxygen ions, or the single corner oxygen ion bonding with two Al ions. In this case aluminum is slightly less electropositive than is silicon and is able to approach close enough that corner oxygen ions can be shared. In a matrix of these octahedral units each oxygen will be bonded to two aluminum ions, leaving it with a remaining -1 charge. The charge can be satisfied by attaching a proton (hydrogen ion) and when this type of structure is continued in three dimentions we have the mineral GIBBSITE.

We have another option for balancing the remaining -1 charge on the oxygen ions. Remember we left a sheet of silicon tetrahedral units with apical oxygen ions still having an unbalanced charge? The two sheets can be brought together with the apical oxygen ions of the tetrahedral layer also being in the octahedral layer. As a result, the charge on these oxygen ions is balanced by bonding to one silicon ion and two aluminum ions. This is the basic structure of our first phyllosilicate mineral, KAOLINITE. The structure illustrated here is about 0.7 nm thick (from the bottom oxygen to the top oxygen) and extends 10 nm and more in the other two directions. We can speak of this three dimensional structure as a clay micelle. A three dimensional kaolinite structure with the Si-tetrahedra and Al-octahedra shown as geometric solids is available for viewing (courtesy of P. Barak, UW-Madison).

The kaolinite mineral is actually made up of many micelles piled one atop the other. Since the surface on one micelle contains hydrogen ions and the other surface only oxygen ions there is a tendency for hydrogen bonds to form between micelles. While individual hydrogen bonds are very low energy, the bonding energy is additive and the sum of the many hydrogen bonds between micelles results in the micelles being very strongly bonded together and nearly impossible to seperate. Thus, we speak of kaolinite as being a nonexpanding phyllosilicate. There is one more item of terminology that should be introduced: since each micelle is constructed of a layer of silicon tetrahedral units and a layer of octahedral units, kaolinite is called a 1:1 clay mineral. Thus, kaolinite is a 1:1 nonexpanding clay mineral.

Now, since we replaced the hydrogen ion on one layer of octahedral oxygen ions by a silicon ion it is only logical that the remainder of the hydrogen ions can be similarly replaced.

Indeed, this can be done as shown at left, and another class of clay minerals is thereby introduced. By the same logic as above, we have now formed the basic structure of 2:1 clay minerals. These minerals consist of two silicon tetrahedral layers and one aluminum octahedral layer. A three dimentional structure of the 2:1 clay minerals with the Si-tetrahedra and Al-octahedra shown as geometric solids is also available for viewing (also courtesy of P. Barak, UW-Madison). There are many minerals with this structure, but for now we will simply refer to them as the Mica group. For a description of how these minerals are built into a crystal go to the Muscovite Mica page.

There is one additional type of mineral we should discuss before leaving this section. In certain situations we find that a 2:1 clay mineral has been crystallized in an environment containing an excess of aluminum. Under these conditions an extra aluminum octahedral layer (Gibbsite) will form between the micelles as illustrated at right. This formation is not random, but will be found as a complete layer in each intermicelle space. Minerals having this characteristic constitute what is known as the CHLORITE group. The aluminum (Gibbsite) layer has the effect of binding the micelles tightly, and this group of minerals is also nonexpanding.

Some see this group of minerals as consisting of alternate 2:1 micelles and gibbsite layers. Those with this view will usually call the chlorites 2:1:1 minerals. Others feel that the gibbsite layer in an integral part of the micelle, and will usually refer to the chlorites as 2:2 minerals. Regardless of what they may be called, however, the repeat structure now consists of the 2:1:1 or 2:2 combination as indicated by the line of the left of the figure.

The oxygen ions in the octahedral layer are in a close packed structure. To understand what this means, visualize a box full of balls all of the same size. If you gently shake the box the balls will settle together until they take up the minimum possible volume. They are now said to be close packed. As we have already seen, the octahedral layer consists of two planes of oxygen ions, each ion of which is bonded with two of the smaller aluminum ions which are located in the space created between the larger oxygen ions when they come together in this close packed structure. As it turns out, meeting the requirement that each oxygen ion share charges with two aluminum ions does not require aluminum occupancy of all possible spaces between the planes of oxygen ions. Rather, as seen at left, extra sites are created when the oxygen and aluminum ions come together. Across the entire octahedral layer there is an average of one empty site created for every two aluminum ions in the structure. This situation presents an alternate possibility for building the mineral. Two trivalent aluminum ions will provide a total of six positive charges, but so will three divalent ions. Thus, during the process of mineral formation the octahedral layer might be filled with divalent ions rather than the aluminum ions we have been using to this point. Most commonly, either divalent iron or magnesium are the ions found replacing the aluminum. If the octahedral layer contains divalent ions in all the possible sites, it is a known as a trioctahedral mineral. If it contains trivalent ions in two of every three possible sites, it is known as a dioctahedral mineral. (Keep this straight! It can be a point of confusion!) If you were able to view the three dimentional structures, both depicted dioctahedral minerals.

Discussion to this point has centered on balancing all charges within the crystal structure, and there are minerals which do have these charges all balanced. There are others, however, which contain charge unbalances within the crystal structure. These minerals are said to contain a permanent charge. We stated at the outset of our discussion that the tetrahedra and octahedra with which the phyllosilicates are built require that the cation:oxygen radius ratio be from 0.255 to 0.414 and from 0.414 to 0.732, respectively. As indicated in the discussion of dioctahedral vs trioctahedral minerals, there are other ions in our environment which fit the criteria. Mineral analysis over the years has indicated that the following ions are found in the phyllosilicates. As seen in the table below there are a variety of ions which can fit into either tetrahedral or octahedral polyhedra. Aluminum is unique in that it falls right on the line between the two. Therefore, it is possible for the trivalent Al to replace some of the quadrivalent Si, resulting in an imbalance of -1 for each substitution. We also at times find divalent ions in the octahedral layer of dioctahedral minerals, and trivalent ions in the octahedral layer of trioctahedral minerals. In these cases there will be, respectively, a negative and positive imbalance of charge.
 

Ions found in Phyllosilicate Minerals

common constituents			Occasional constituents			Cations found in interlayer spaces
ion	radius (nm)	rc/ro	ion	radius (nm)	rc/ro	ion	radius (nm)	rc/ro
O2-	0.135	-	Ni2+	0.074	0.55	Na+	0.101	0.75
Si4+	0.040	0.30	Ti4+	0.060	0.44	K+	0.134	1.00
Al3+	0.055	0.41	Zn2+	0.057	0.42	Cs+	0.163	1.24
Fe2+	0.080	0.59	Mn2+	0.083	0.61	Ca2+	0.105	0.78
Fe3+	0.067	0.54	Mn3+	0.072	0.53	Ba2+	0.140	1.03
Mg2+	0.078	0.58	Mn4+	0.052	0.39	Sr2+	0.118	0.87
.			Li+	0.076	0.56	H2O	0.145	..
			Cr3+	0.065	0.48	NH4+	0.143	.
			Cu+	0.095	0.70	..

A second source of charge on the minerals is the broken bonds found at the mineral edges. The structure cannot extend infinitely, so at some point there will be oxygens without all charges satisfied by associating with cations. In these cases a hydrogen ion from solution will normally satisfy the requirement. Whether this can occur will, however, depend on the solution pH. Therefore, these charges are called either pH-dependent charge or variable charge.

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