Chapter 4 Minerals Sec. 4.1 What is a Mineral?
Minerals Earth s crust is composed of about 3000 minerals. Besides forming rocks and shaping Earth s surface, some minerals have helped to develop civilization. A mineral is a naturally occurring, inorganic solid, with a special chemical composition and a definite crystalline structure. A definite crystal structure means minerals are arranged in regular repeating patterns, many times creating a crystal. Commonly, the internal structure of a mineral is not apparent due to the restricted space in which it formed. Crystals which had room to grow large are quite rare. A synthetic substance, such as diamonds and others, are not minerals. Therefore, coal and sugar, formed from organic materials, are not considered as minerals. By definition, minerals are solids that have definite shapes and volumes; no gas or liquid can be considered a mineral. All minerals are composed of a specific chemical composition, but some are composed of a single element, such as copper, silver, sulfur.
Variations in Composition In some minerals, chemical composition can vary slightly depending on the temperature at which the mineral crystallized. See pg. 88, Fig. 4.3 Plagioclase feldspar ranges from sodium-rich albite at low temperatures, to calcium-rich anorthite at high temperatures. At intermediate temperatures, both calcium and sodium are incorporated into the crystal structure, which accounts for the refractions of light that produce a range of colors.
Rock-Forming Minerals Although there are about 3000 minerals in Earth s crust, only about 30 of these are common; and only about 8-10 of those are referred to as rock-forming minerals. Minerals from Magma Magma is hotter and less dense than surrounding rock, so it can rise up into cooler layers of Earth s interior. The type and number of elements in the magma determines which minerals will form. If magma cools slowly, the crystals have time to arrange themselves into large crystals. If magma cools quickly, smaller crystals form and many times run into each other forming a rock of irregular colors and disordered crystal structure.
Minerals from Solutions Minerals are often dissolved in water. When a liquid becomes full of that substance and can dissolve no more of it, the solution is said to have become saturated. If the solution becomes overfilled, it is called supersaturated and conditions are right for crystals to form. Then, individual atoms can form bonds and mineral crystals precipitate, forming solids in the solution. Evaporites can form as mineral-rich water evaporates, causing the minerals to precipitate from the solution. Rock salt and many cave features form in this way.
Identifying Minerals Geologists rely on a number of tests to identify minerals. It is best to rely on a series of tests in order to identify minerals. Crystal form Some shapes are immediately recognizable. Quartz forms pointed hexagons; halite forms cubic crystals. Luster The way a crystal reflects light form its surface. Metallic, non-metallic, waxy, pearly, dull, silky, or earthy. This test is quite subjective; people don t always agree. Hardness A very useful way to help ID minerals. This can range from minerals that can be scratched with your finger nail to diamond, the hardest substance known. Cleavage & Fracture Minerals that break easily along a surface are said to have cleavage. These minerals split along weak atomic bonds. Fracture describes an irregular or curved break, much like glass. This is called a conchoidal fracture. Streak the color of a mineral when it is broken up and powdered. This is usually done by rubbing the mineral against a porcelain plate. Although the same mineral make have different appearances, the streak will always be the same color.
Identifying Minerals (cont.) Special Properties These can be helpful in identifying minerals. Included are properties like magnetism, striations, double refraction, & effervescense with HCL, and fluorescence. (See pg. 94, Table 4.3) Color This is one of the least reliable of properties. In contrast, knowing the variety of colors a particular mineral may take may help in identifying the specimen. Texture This describes how a mineral feels to the touch. This, like luster, is very subjective, so it is best used along with other tests. Textures can be described as smooth, ragged, rough, greasy, or soapy. Density Sometimes, two minerals of the same size have different rates. This is due to a difference in density, which reflects the atomic mass & structure of a mineral. Gold has a higher density than pyrite. If you had a piece of both of the same size, the gold would be much heavier. Density = Mass Volume
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