Van der Waals Bonding in Minerals

Van der Waals bonding is a very weak intermolecular attraction that, like hydrogen bonding, depends on an asymmetric distribution of electrical charge - but it arises by an entirely different mechanism. ^[1] Rather than a permanent polarity built into the structure of a molecule, Van der Waals bonding is driven by the instantaneous fluctuations in electron positions that occur continuously within any atom or bonded group of atoms. These fluctuations are transient, but they are enough to generate short-lived regions of unequal charge that attract neighboring atoms or layers.

The Polarization Mechanism

Graphite provides the clearest illustration of how Van der Waals bonding works. ^[1] Within each sheet of carbon atoms, the electrons are shared in covalent bonds and their time-averaged distribution is symmetric: integrated over any reasonable period of time, the charge on both sides of the sheet is equal. At any given instant, however, electrons are not perfectly distributed - more will be concentrated on one side of the sheet than the other, producing a polarization (Figure 3.14) - positive on one side and negative on the other. ^[1] This net positive charge on one side attracts electrons from the adjacent sheet, causing it to become polarized as well. ^[1] The aligned opposite charges on facing surfaces of adjacent sheets generate the weak electrostatic attraction that is the Van der Waals bond. ^[1]

Properties of Van der Waals-Bonded Minerals

Because Van der Waals forces are the weakest of all bonding mechanisms in minerals, any mineral held together by these bonds between its structural units will be very soft and will often have a characteristic greasy or slippery feel in hand specimen. ^[1] Graphite (hardness 1-2) and talc (hardness 1) are the most familiar examples. The softness arises because so little force is needed to overcome the Van der Waals attraction between structural units - in graphite’s case, between the carbon sheets; in talc’s case, between the silicate layers.

Mineral Examples

Graphite is soft and black and is widely used to make pencil lead; the ease with which its carbon sheets slide across each other also makes it an effective lubricant. ^[1] These practical properties follow directly from the Van der Waals bonds between sheets: the sheets move past each other with very little resistance precisely because the interlayer attraction is so weak.

Talc [Mg₃Si₄O₁₀(OH)₂] finds widespread application in cosmetics and body powders precisely because its extreme softness prevents irritation and chafing on sensitive skin. ^[1]

The same weakness of Van der Waals and hydrogen bonding in clay minerals creates a significant engineering problem: swelling soil, in which clay-rich ground absorbs water and expands, damaging foundations and infrastructure. ^[1]

Multiple Bonding Types in One Mineral

An important point illustrated by graphite is that a single mineral may contain several fundamentally different types of chemical bonds operating simultaneously. ^[1] Within each carbon sheet, the atoms are linked by strong covalent sigma bonds and by pi bonds that have an intermediate covalent-metallic character; between sheets, only Van der Waals bonds act. This hierarchy of bond strengths - strong within layers, negligible between them - produces the dramatic mechanical anisotropy that makes graphite simultaneously hard to cut across the layers and trivially easy to separate along them.

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