ORE MINERALS

ORE MINERALS

Figure 3–12 Sapphire is one of the most costly precious Ore minerals are minerals from which metals or other

gems. (Smithsonian Institution) elements can be profitably recovered. A few, such as native gold and native silver, are composed of a single element. However, most metals are chemically bonded to anions.

with very similar chemical compositions, crystalline Copper, lead, and zinc are commonly bonded to sulfur to structures, and appearances. The rock-forming minerals

form the important ore minerals chalcopyrite, galena are described in Section 3.6.

(Fig. 3–14), and sphalerite.

ACCESSORY MINERALS

INDUSTRIAL MINERALS

Accessory minerals are minerals that are common but Several minerals are industrially important, although usually are found only in small amounts. Chlorite, gar-

they are not considered ore because they are mined for net, hematite, limonite, magnetite, and pyrite are com-

purposes other than the extraction of metals. Halite is mon accessory minerals (Fig. 3–11).

mined for table salt, and gypsum is mined as the raw ma- terial for plaster and sheetrock. Apatite and other phos- phorus minerals are sources of the phosphate fertilizers crucial to modern agriculture. Many limestones are made up of nearly pure calcite and are mined as the raw mate- rial of cement.

Figure 3–14 Galena is the most important ore of lead and Figure 3–13 Topaz is a popular semiprecious gem.

commonly contains silver. (Ward’s Natural Science Establish- (American Museum of Natural History)

ment, Inc.)

Mineral Classification 49

3.6 MINERAL CLASSIFICATION

characteristics of graphite and diamond emphasize the importance of crystalline structure in determining the

Geologists classify minerals according to their anions physical properties of minerals. (negatively charged ions). Anions can be either simple or complex. A simple anion is a single negatively charged ion, such as O 2⫺ . Alternatively, two or more atoms can

OXIDES

bond firmly together and acquire a negative charge The oxides are a large group of minerals in which oxy- to form a complex anion. Two common examples are

gen is combined with one or more metals. Oxide miner-

als are the most important ores of iron, manganese, tin, anions.

the silicate, (SiO 4 ) 4⫺ , and carbonate, (CO 3 ) 2⫺ , complex

chromium, uranium, titanium, and several other indus- Each mineral group (except the native elements)

trial metals. Hematite (iron oxide, Fe 2 O 3 ) occurs widely is named for its anion. For example, the oxides all con-

in many types of rocks and is the most abundant ore of

iron. Although typically red in color, it occasionally oc- ates contain (CO 3 ) 2⫺ .

tain O 2⫺ , the silicates contain (SiO 4 ) 4⫺ , and the carbon-

curs as black crystals used as semiprecious gems. Magnetite (Fe 3 O 4 ), a naturally magnetic iron oxide, is another ore of iron. Spinel (MgAl O NATIVE ELEMENTS ) often occurs as at- 2 4 tractive red or blue crystals that are used as inexpensive, About 20 elements occur naturally in their native states

semiprecious gems. Synthetic spinels are also commonly as minerals. Fewer than ten, however, are common

used in jewelry. Ice, the oxide of hydrogen (H 2 O), is a enough to be of economic importance. Gold, silver, plat-

common mineral at the Earth’s surface. inum, and copper are all mined in their pure forms. Iron is rarely found in its native state in the Earth’s crust, but metallic iron is common in certain types of meteorites.

SULFIDES

Native iron and nickel are thought to comprise most of Sulfide minerals consist of sulfur combined with one or the Earth’s core. Native sulfur, used to manufacture sul-

more metals. Many sulfides are extremely important ore furic acid, insecticides, fertilizer, and rubber, is mined

minerals. They are the world’s major sources of copper, from volcanic craters, where it is deposited from gases

lead, zinc, molybdenum, silver, cobalt, mercury, nickel, emanating from the vents (Fig. 3–15).

and several other metals. The most common sulfides are Pure carbon occurs as both graphite and diamond.

pyrite (FeS 2 ), chalcopyrite (CuFeS 2 ), galena (PbS), and The minerals have identical compositions but different

sphalerite (ZnS).

crystalline structures and are called polymorphs, after the ancient Greek for “several forms.” Graphite is one of the softest minerals and is opaque and an electrical

SULFATES

conductor. Diamond, the hardest mineral known, is The sulfate minerals contain the sulfate complex anion transparent and an electrical insulator. The contrasting (SO 4 ) 2⫺ . Gypsum (CaSO 4 ⭈ 2H 2 O) and anhydrite

(CaSO 4 ) are two important industrial sulfates used to manufacture plaster and sheetrock. Both form by evapo- ration of seawater or salty lake water.