Earth Science: Origins and Geology – Minerals – Structure
Minerals—the basic ingredients of rock-provide many of our most important raw materials. They usually occur in the form of crystals, which have been treasured throughout history for their colorful variety and refractive properties.
Crystals are solid substances whose ions, atoms, or molecules are arranged in a repetitive three-dimensional lattice structure. The configuration of the lattice depends on the mineral’s chemical composition. Only a few minerals, such as the precious stone opal, are not found in crystalline form; these are called amorphous minerals. Polymorphic minerals, on the other hand, share the same chemical composition but exhibit different crystal structures.
For instance, diamond and graphite both result from carbon. Diamond-the hardest mineral in nature-forms in an environment of high pressure and temperature, while soft graphite arises under low pressure and temperature conditions. Thanks to this realization, diamonds can be artifically produced today. Synthetic crystals are also manufactured in the semiconductor industry, whose most important raw material is silicon.
Aside from their classification into crystal systems, minerals can be distinguished based on their physical properties, such as hardness or density. Hardness is the measurement of a substance’s resistance to scratching. Density, or specific weight, depends on its constituent elements and how closely the atoms within it are packed. When a mineral cracks, the breaks generally follow its atomic lattice structure. Thus mica, rock salt, and calcite tend to crack into pieces with flat, parallel surfaces. With quartz, on the other hand, the bonds between the atoms are so strong that the broken pieces are irregular.
The most obvious characteristics of minerals are their color and shininess. Some are also distinguished by their magnetic, fluorescent, or radioactive properties, as well as the conditions under which they formed. Crystals may exhibit various types of symmetry. On this basis, they are divided into 32 classes, which in turn are assigned to seven different crystal systems: the cubic, hexagonal, rhombohedral (or trigonal), tetragonal, orthorhombic (or rhombic), monoclinic, and triclinic systems. The cubic system, such as table salt, rock salt, galena, and pyrite) is the easiest to recognize.
GEMSTONES
Precious stones are minerals used as jewelry. Whether an amethyst, rose quartz, aquamarine, or garnet is used as jewelry depends on its purity, color, and translucency. Weight too plays a significant role for the value of such pieces.
Diamonds are among the most precious gem- stones although emeralds, rubies, and sapphires are also popular. In the past these stones were usually worn as coarse or roundly polished forms; however, nowadays many gemstones are polished in crystal-shaped forms, in order to enhance the reflection of light and the luster.
BASICS
HARDNESS OF A MINERAL is defined as the resistance it displays during mechanical stress. In particular, the scratch resistance is closely compared. On the ten point Mohs’ Hardness Scale (named after the German mineralogist Friedrich Mohs). a mineral can scratch any other mineral with a lower or same hardness number.
Talcum, for example, has the lowest degree of hardness while a diamond has the highest; therefore, a diamond can only be scratched by another diamond.