Minerals and history, Branches of mineralogy
Mineralogy is the branch of geology concerned with the study of minerals. A mineral is a naturally occurring, homogeneous solid with a definite chemical composition and a highly ordered atomic structure. A homogeneous substance is one that can be divided into repeating units that are exactly the same. A mineral, by definition, cannot be a liquid or a gas. The chemical composition of a mineral is definite, meaning a particular mineral is always composed of the same ratio of elements, and this composition can be shown using a chemical formula. The atoms in a mineral are arranged in a highly ordered fashion, called a crystal lattice structure.
Minerals have been an important part of our society since the time of prehistoric man. Early humans carved tools out of minerals such as quartz. Pottery has been made of various clays since ancient times. Sodium chloride, also known as the mineral halite, has been used in food preservation techniques for millions of years. Mining of useful minerals out of ores became widespread hundreds of years ago, a practice still in use today.
There are several different branches of mineralogy. Mineralogists can focus on very specific studies, from crystal structure to classification or chemical composition. Crystallography, for example, is the study of the crystal lattice structure of minerals. As mentioned above, the atoms in a mineral are arranged in a highly ordered fashion. This ordered arrangement produces crystals of definite size and shape. A particular mineral sample is made up of repeating crystal units. Each crystal that makes up the mineral has the same shape. There are six basic shapes a mineral crystal can have. The shape of the crystal, as well as how tightly packed the atoms are in the crystal, help determine the physical properties of the mineral. Crystals that are allowed to grow with plenty of open space will form nearly perfect structures, and those that form in more cramped conditions will display imperfections in the crystal shape.
Crystal and conformational chemistry
Crystal chemistry is the branch of mineralogy that deals with how the chemical composition of a mineral relates to its crystal structure. The chemical bonds formed between atoms determine the crystal shape as well as the chemical and physical properties of the mineral. There are three different types of chemical bonds present in minerals—ionic, covalent, and metallic. In ionic bonding, an atom with a positive charge binds to an atom with a negative charge through electrostatic attraction. Minerals with ionic bonds tend to be poor conductors of heat and electricity, have low melting points, and are brittle. Halite and fluorite are both minerals formed by ionic bonds. In covalent bonding, electrons are shared between two atoms. This type of bonding is stronger than ionic bonding, which means minerals with covalent bonds have higher melting points and are harder than those with ionic bonds. These minerals are also poor conductors of heat and electricity and are brittle. Examples of covalently bonded minerals include quartz and diamond. Metallic bonding occurs between atoms of metals. In this type of bond, the outer electrons of the atom are free to move, and are shared between all of the other atoms in the substance. This special structure is the reason metals are good conductors of heat and electricity, are malleable, soft, and have lower melting points. Copper, silver, and gold are all minerals formed by metallic bonding.
Physical mineralogy is concerned with the physical properties and descriptions of minerals. Minerals can be described using several physical attributes, including hardness, specific gravity, luster, color, streak, and cleavage.
The hardness of a mineral can be determined by a scratch test. The scratch test establishes how easily a mark can be made on a mineral sample using different materials. If a mark is made easily, the mineral is not very hard. If no mark can be made, then the mineral is quite hard. The hardness is then measured on a scale of 1-10, called Mohs' hardness scale, named after the Austrian scientist F. Mohs, who developed this procedure. If a fingernail can scratch a particular mineral, it would have a hardness of 2.5. If a penny can scratch it, its hardness is around 3. If a mineral can be scratched by glass, its hardness is 5.5. If it can be scratched by unglazed porcelain, it has a hardness between 6 and 6.5, and if a steel file can leave a mark, it has a hardness of 6-7. Talc is the softest mineral with a hardness rating of 1, while diamond is the hardest, rated 10.
The luster of a mineral is the appearance of its surface when light is reflected off of it. Minerals can have metallic or nonmetallic luster. Minerals with metallic luster look shiny like a metal. Nonmetallic minerals can have various appearances, such as vitreous (glassy), greasy, silky, brilliant (like a diamond), or pearly.
The color of a mineral sample cannot be used to definitively identify the mineral because of impurities that may be present, however, the color can narrow down the identity of a mineral to a few choices. The streak of a mineral is the color of its powdered form. Rubbing the mineral across an unglazed porcelain square, called a streak plate, can best show streak color. A mineral will have a characteristic streak color, although more than one mineral may have the same color. Therefore, streak is not a definitive identification tool, although it may be used to verify the identity of a mineral of suspected composition.
A mineral exhibits cleavage when it breaks along a certain direction or plane, producing a flat surface along the break. When a mineral shatters, rather than breaks along planes, it exhibits fracture. Cleavage is characteristic of particular minerals such as feldspar, while minerals such as quartz show fracture. Each of these physical properties can be used to determine the chemical identity of an unknown mineral, and together are the focus of the branch of mineralogy called physical mineralogy.
Descriptive mineralogists use the properties discussed in physical mineralogy to name and classify new minerals. Determinative mineralogy is the branch of mineralogy that deals with identifying unknown minerals, also using the physical properties of minerals. Other branches of mineralogy include chemical mineralogy (identifying minerals to determine the chemical composition of the earth's crust), optical mineralogy (using light to determine the crystal structure of minerals), xray mineralogy (using x-ray diffraction techniques to determine the crystal structure of minerals), and economic mineralogy (the study of new, economically important uses for minerals). All of the branches of mineralogy together describe the physical and chemical properties of minerals and their uses.
Mineralogy is an important discipline for several reasons. For one, the study of the composition of the earth's crust gives scientists an idea of how Earth was formed. The discovery of new minerals could provide useful materials for industry. The study of the chemical properties of minerals could lead to the discovery of new uses for Earth's mineral resources. Mining ores for their mineral components provides the materials for lasers, buildings, and jewelry. Each of the branches of mineralogy contributes to the indispensable knowledge base of minerals and their uses.
Klein, C. The Manual of Mineral Science. 22nd ed. New York: John Wiley & Sons, Inc., 2002.
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