Mineralogy 101: Crystals

In an early article I covered some of the basic concepts and methods used for mineral identification. (See my article on Mineral Identification for more information.) While my earlier article provides useful information for mineral identification, it did not go into depth on several aspects of minerals and their physical properties. Over the coming year I plan to rectify this through several articles (written periodically) that will go into more detail on minerals, their formation, and internal structure.

To really study and understand how minerals form, and to aid us in classifying and identifying them we need to understand their crystal structure. Crystals, as defined by most mineralogists are regular geometric solids bounded by smooth plane surfaces. Many times the term crystal is used to describe a perfectly developed mineral, so geologists also use other terms to help classify crystals. A crystal with well-formed faces is called euhedral. Those with imperfect faces are subhedral, and those without faces are anhedral. Sometimes minerals will form that do not have an organized internal structure and are considered amorphous. Amorphous substances that occur naturally are called mineraloids, the best known being opal.

Crystals can form in one of three methods, either from a solution, melts, or vapors. Crystals from solutions form as the temperature or pressure of the solution is lowered. When a solution is under pressure, or at a high temperature, it can hold greater concentrations of the elements that will crystallize, and as the temperature or pressure lowers, the elements become supersaturated in the solution and crystals will form. A good example of this is the growing of salt crystals from hot water supersaturated with salt. As the salt-water solution cools, the salt crystals will grow.

Crystals can form from melts in the same way that they form from solutions. As the melt cools, the molecules that will form the crystals become attracted to each other to form the different minerals. This is the method by which most of the minerals associated with igneous rocks are formed. A good example for crystals forming from melts is how ice forms when water freezes.

Crystals that form from vapor are not as common, but follow the same principles. As the vapor that contains the molecules cools, they are brought closer together, eventually forming into a crystalline solid. A great example of this is how water vapor will form snowflakes as the temperature cools.

Crystals all have a well-ordered internal structure, and the external faces of the crystal reflect this structure. To help understand the terminology and how crystals are classified, we need to examine how the different patterns that are the building blocks of the crystals can be arranged. (The patterns are the elements that combine to form the crystal.) There are three basic ways by which patterns in crystals can form. (The diagram at the right demonstrates these basic patterns.) The first is called translation, and it generates a linear pattern at intervals equal to the distance, t. Rotation occurs through an angle (a) and creates a pattern along the path of a circle. Combining these two patterns, translation with rotation, will create a corkscrew pattern.

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