Minerals and their Physical Properties

This week our lab is on the identification of minerals using their physical properties (lab handout, mineral chart, flow chart.) Minerals are an important natural resource. Everything we use that is not grown, is derived from the Earth. These resources include rocks, minerals, and hydrocarbons to name a few. Therefore, understanding where to find these resources is rather important. Common mineral uses include (from http://www.mii.org/commonminerals.html):

1. Gypsum:  Processed and used as prefabricated wallboard or as industrial or building plaster, used in cement manufacture, agriculture and other uses.
2. Feldspar: A rock-forming mineral, industrially important in glass and ceramic industries, pottery and enamelware, soaps, abrasives, bond for abrasive wheels, cements and concretes, insulating compositions, fertilizer, poultry grit, tarred roofing materials, and as a sizing (or filler) in textiles and paper. Albite is a feldspar mineral and is a sodium aluminum silicate. This form of feldspar is used as a glaze in ceramics.
3. Fluorite (fluorspar): Used in production of hydrofluoric acid, which is used in the electroplating, stainless steel, refrigerant, and plastics industries, in production of aluminum fluoride, which is used in aluminum smelting, as a flux in ceramics and glass, and in steel furnaces, and in emery wheels, optics, and welding rods.
4. Halite (Sodium chloride--Salt): Used in human and animal diet, food seasoning and food preservation, used to prepare sodium hydroxide, soda ash, caustic soda, hydrochloric acid, chlorine, metallic sodium, used in ceramic glazes, metallurgy, curing of hides, mineral waters, soap manufacture, home water softeners, highway deicing, photography, herbicide, fire extinguishing, nuclear reactors, mouthwash, medicine (heat exhaustion), in scientific equipment for optical parts. Single crystals used for spectroscopy, ultraviolet and infrared transmission.
5. Kaolinite: Also known as "china clay" is a white, aluminosilicate widely used in paints, refractories, plastics, sanitary wares, fiberglass, adhesives, ceramics, and rubber products.
6. Galena: A lead sulfide, the commonest ore of lead.
7. Mica: Micas (muscovite and biotite) commonly occur as flakes, books, or sheets. Sheet muscovite (white) mica is used in electronic insulators (mainly in vacuum tubes), ground mica in paint, as joint cement, as a dusting agent, in well-drilling muds, and in plastics, roofing, rubber, and welding rods. 
8. Pyrite: Used in the manufacture of sulfuric acid and sulfur dioxide; pellets of pressed pyrite dust have been used to recover iron, gold, copper, cobalt, nickel, etc.; used to make inexpensive jewelry.
9. Quartz (Silica): As a crystal, quartz is used as a semiprecious gem stone. Cryptocrystalline forms may also be gem stones: agate, jasper, onyx, carnelian, chalcedony, etc. Crystalline gem varieties include amethyst, citrine, rose quartz, smoky quartz, etc. Because of its piezoelectric properties quartz is used for pressure gauges, oscillators, resonators, and wave stabilizers; because of its ability to rotate the plane of polarization of light and its transparency in ultraviolet rays it is used in heat-ray lamps, prism, and spectrographic lenses. Used in the manufacture of glass, paints, abrasives, refractories, and precision instruments.
10. Sulfur: Used in the manufacture of sulfuric acid, fertilizers, chemicals, explosives, dyestuffs, petroleum refining, vulcanization of rubber, fungicides.
11. Hematite (iron ore): About 98% of iron ore is used to make steel— one of the greatest inventions and most useful materials ever created. While the other uses for iron ore and iron are only a very small amount of the consumption, they provide excellent examples of the ingenuity and the multitude of uses that man can create from our natural resources.    

Another important aspect of these resources is that they are finite. The rock, mineral, and coal resources are removed from mines and when they are gone, they are gone. The environmental damage that results from their removal, however, remains.

The Coeur Rochester mine in the Humboldt River Basin is the largest operating silver mine in the conterminous United States. The silver-rich ores are being mined with open-pit methods, which is a common technique for mining silver and gold in the region. In 2003, mining in Nevada produced $50 million of silver (56% of U.S. consumption) and $2.7 billion of gold (82% of U.S. production). (USGS photograph by Alan R. Wallace.)

Mountain top removal form of open pit mining in West Virginia. The rock above the coal is removed from the mountain and dumped in nearby valleys. (photograph by Denny Tyler)

This week we will discuss minerals: the building blocks of rocks. A mineral is a naturally occurring, inorganic, crystalline solid with diagnostic physical properties and a definite chemical composition. Breaking that definition down: naturally occurring means that the mineral was formed through geological processes, not by people; inorganic means that the compound is inorganic, that is, it is not composed of carbon and hydrogen; and, crystalline solid means that the compound is solid and possesses an ordered internal structure. We use the mineral's physical properties to identify the mineral. These physical properties are consistent from sample to sample and include:

• color: the wavelength of light reflected from the mineral.
• luster: the quality and intensity of light reflected from the mineral. The most notable lusters are metallic and nonmetallic. There are a variety of nonmetallic lusters including pearly, glassy, vitreous, dull, and earthy.
• streak: the color of the powdered form of the mineral.
• hardness: the ability of a mineral to resist abrasion. Think "scratchability."

• cleavage: the ability of a mineral to break along planes of weakness in the crystal lattice.

Examples of cleavage. (Image from Geoscience Laboratory Manual by Tom Freeman.)

• crystal form: the external reflection of the internal structure of a mineral

Mass of quartz crystals. Quartz is composed of silica tetrahedra only. These tetrahedra are arranged in a 3-D framework. (Image courtesy of the USDA)

Fluorite has cubic crystal form however, it has octahedral cleavage, it breaks in to diamond-like figures.

Ice crystal. Ice meets the definition of mineral even though it's transient presence on the surface of the earth makes it seem an unlikely candidate. (Image courtesy of USDA Beltsville Agriculture Research Center)

• specific gravity: the ratio of the density of a mineral to the density of water as shown below.

                             density of mineral in grams per cubic cm

                               density of water in grams per cubic cm

• special properties: magnetite is magnetic, halite tastes salty, sulfur smells bad

There are many good websites with more information about the physical properties of minerals. A good one for hardness is located here.

The chemical composition of a mineral is narrowly defined. There can be some substitutions of cations but only within a narrow range. For example: olivine is composed of iron and/or magnesium plus the silicate ion: (Fe, Mg)₂SiO₄. Although there are two possible cations in olivine, only iron or magnesium will fit in the crystal lattice. Therefore, substitution is possible in some minerals but the options for an individual mineral are limited.

Approximately 3000 minerals have been identified around the world. However, only about 25 are common in rocks. These include the following minerals in our mineral boxes:

1. Plagioclase feldspar
2. Potassium feldspar (K-spar)
3. Quartz
4. Muscovite
5. Olivine
6. Pyroxene
7. Hornblende (a type of Amphibole)
8. Biotite
9. Chlorite
10. Calcite
11. Iron oxides such as hematite
12. Clay minerals such as kaolinite

Some minerals are only found in one location in the world. Others are found only in certain tectonic environments. Still others are so common, they are in nearly everyone's backyard. Knowing the minerals in the above list, will allow one to know at least one mineral in nearly every rock he or she picks up anywhere in the world.

A photograph of Blue John, a variety of flourite found only in Castleton, Derbyshire, England, UK. Blue John is used to make ornaments and jewlery and has been mined since 1762. Today, only 500 kg of Blue John are mined per year. (photo from www.mineral-forum.com)

Regular examination of the minerals in the boxes and in the rocks we study will make identification of the minerals you are expected to know much easier. There is only one way to learn the minerals and that is to study them in person. Learn which physical properties are useful and how to use the tools to test for each physical property. The key to the mineral lab can be found here: mineral lab key (will be posted when we have finished the lab.)