Crystallographic Mineralogy and Crystal Formations

A crystal is formed from a fused in much the same way as from a solution. The most familiar example of crystallization from fusion is the formation of ice crystals when water freezes. While we do not ordinarily consider it in this way, water is fused ice. When the temperature is sufficiently lowered the water can no longer remain liquid, and it becomes solid by crystallization into ice. The particles of water which were free to move in any direction in the liquid now become fixed in their position, and by the laws of molecular attraction arrange themselves in a definite order and built up a solid crystalline mass. The formation of igneous rocks from molten lavas, while more complicated, is similar to the freezing of water. In the fluid lava we have many elements in a dissociated state. As the lava cools these elements gradually group themselves into different mineral molecules, which gather together and slowly crystallize to form the mineral particles of the resulting solid rock.

The third mode of crystal formation, that in which the crystals are produced from a vapor, is less common than the other two described above. The principles that underlie the crystallization are much the same. The dissociated chemical atoms through the cooling of the gas are brought closer together until they at last form a solid with a definite crystal structure. An example of this mode of crystal formation is seen in the formation of sulfur crystals about the mouths of fumaroles in volcanic regions, where they have been crystallized from sulfur-bearing vapors.

The most fundamental and important fact converting crystals is that they possess a definite internal structure. A crystal is to be conceived as made up of an almost infinite number of excessively minute chemical particles which have a regular arrangement and relation to each other and form, as it were, a crystal network. It has been possible recently to determine in many cases, not only the kind of atomic structure that a given substance possesses, but to place in that structure the positions of the various kinds of atoms present. The unit of structure must be considered as the smallest portion of the substance that still possesses its characteristic properties. This can never be as small as an individual atom since the relations of the atoms to each other are important factors in determining the characters of the substance. It must therefore consist of a group of atoms sufficiently large to show all the properties of structure of the crystal, or a chemical molecule, or a group of molecules. For instance in quartz the structural unit has been shown to consist of three molecules of structural unit has been shown to consist of three molecules of SiO2- Any smaller subdivision would not have the properties of the mineral. There are many proofs that a crystal does possess a definite internal structure but the following are the most important.