The structure of crystals. The crystalline state
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It is well known that matter is formed by atoms (and/or ions) that sometimes group together to form what we know as molecules. Historically, these aggregates (atoms, ions or molecules) have been classified in the three fundamental states of matter: gas, liquid and solid. The so-called crystalline state corresponds fundamentally to the solid state, but with very special characteristics.

Matter is considered strictly crystalline if the atoms, ions or molecules that compose it are distributed in the three independent directions of space in a regular and repetitive way. This form of matter is called crystal, a word (κρύσταλλος, crustallos, or phonetically Kroos'-tal-los = cold + drop) that ancient Greeks identified with the mineral quartz, defining it as icicles of extraordinary hardness and very cold. Crystals usually show their internal order by the appearance of regular external morphologies, that is, through perceptible faces and edges, as it is seen in the two samples shown below.

Quarzt crystalBoleite
Crystals of quartz and boleite (a complex halide mineral)

Crystal packing
A crystalline packing of molecules, showing its highly ordered structure that extends in all directions

Although the concept of crystal is usually associated with the solid state of minerals, biological molecules such as proteins give rise to extremely fragile crystals, with water contents reaching up to 80% of the crystal volume, representing a state much closer to the liquid than to the solid. However, they generally show an external morphology that reveals their internal order.
Protein crystals
Protein crystals. Up of 80% of their volume is water!!!

In another sense the crystalline state can also be considered as a limit of the evolution of the liquid state towards solid. In the liquid state the molecules, very close to each other, come into contact and retain a certain attraction with their neighbors, and hence when we transfer a liquid from one container to another, it maintains a constant volume. However, in part due to the thermal agitation, intermolecular contacts are not stable enough to maintain the volume of the liquid rigid. If we reduce the thermal agitation the fragile contacts (or bonds) between molecules will become more and more stable, reaching a relatively rigid state. If the temperature decrease has been slow enough, the molecules pack together in an orderly manner that corresponds to the most stable possible situation, the one with the lowest energy. This is the crystalline state.

The water molecules in the liquid state attract each other through a special type of dipole-dipole interaction known as hydrogen bonding. The molecules undergo rapid thermal motions, so the lifetime of any specific clustered configuration is very short

The ordered crystal structure of ice. Its three-dimensional structure is maintained by stable hydrogen bonds

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