Description of Minerals and Rocks

The terms mineral and rock are often confused. They are frequently used together, and the materials they describe are closely related. In general, a mineral is a naturally occurring, inorganic, chemical element or a group of elements (a compound). A mineral has a crystalline structure and a unique chemical composition. A rock is a mixture of particles or grains of one or more minerals.

When we refer to mineral resources or industrial minerals, however, we generally include materials that are technically rocks—such as limestone, dolomite, and shale. We also include coal and oil, which are in fact organic substances. Conversely, we include in the rock category high-purity sandstone, which is composed almost entirely of quartz, and high-purity limestone, which is composed largely of the mineral calcite.

Mineral Groups

A few minerals are composed of only one element, such as diamond (carbon), native copper, and sulfur, but most minerals are chemical compounds that contain several elements (table 1). Only about 100 of the more than 2,000 identified minerals are considered to be common minerals. Of those 100, fewer than 20 are found everywhere in the Earth's crust. Scientists group minerals according to their chemical composition and crystal structure. The main mineral groups, their composition, and examples of each are shown in table 1.

Rock Types

Rocks, because they are mixtures of minerals, are more complex and are classified according to how they formed. The broadest grouping of rocks is based on the origin of the rock rather than on the minerals that compose it. In this scheme, all rocks are divided into three general groups: igneous, sedimentary, and metamorphic rocks (figure 6).

FIGURE 6: The three main groups of rocks are the processes that create them are shown. Each rock group may be derived from the other two by these processes. The white arrows  indicate alternative paths within the rock cycle.

Igneous Rocks

Igneous rocks can be categorized based on their texture and composition. Igneous rocks are produced directly from hot molten rock, called magma, that comes from deep within the Earth. As magma cools, the elements that form individual minerals come together and crystallize, as water crystallizes into ice on a winter day.

Different minerals crystallize at different temperatures, which causes differences in grain size, or texture. If the magma cools slowly, some individual mineral grains have the opportunity to grow larger than others. If the magma cools quickly, as lava or basalt does, the separate mineral grains are small. Igneous rocks are classified on the basis of the size and arrangement of the individual crystals and the kinds of minerals present (table 2).

When magma reaches the Earth's surface, cooling occurs quickly, and small crystals form. These igneous rocks are called extrusive rocks because they are extruded onto the exterior of the Earth. When magma remains inside the Earth, cooling occurs more slowly, and large crystals form. Such igneous rocks are called intrusive rocks because they form in the Earth's interior. Porphyritic rocks are igneous rocks that have large crystals, called phenocrysts, within a small crystal ground mass. The magma that produced these rocks began to form larger crystals within the Earth, followed by rapid cooling as the magma moved upward or extruded onto the surface, forming the fine-grained surrounding rock.

Igneous rocks can also be classified by composition into felsic and mafi rocks. Felsic igneous rocks are made up primarily of feldspar and quartz (silica). Mafic igneous rocks are made up primarily of minerals rich in iron and magnesium. Felsic rocks are usually light to intermediate in color, and mafic igneous rocks are usually dark.

Sedimentary Rocks

Most of the rocks native to Illinois are sedimentary rocks. Some are made of small pieces of shells, plant and animal remains, and weathered fragments of other rocks (sediments) that have been moved by rivers, waves, winds, or ice (glaciers). These sediments have been deposited and later compacted or cemented by the mineral matter that precipitated out of the water moving through the voids between the sediment particles. Pores are voids in the sedimentary rocks that were not filled with material. Such sedimentary rocks are called clastic (meaning broken pieces) rocks. Other sedimentary rocks may be formed by chemical precipitation from water (chemical) or consist of fossil remains (organic).

Clastic sedimentary rocks are classified by (1) their grain size or texture (examples include gravel, sand, silt, and clay) and (2) their mineral composition (table 3). Rocks such as gypsum and some limestones were formed by chemical precipitation from sea water. Organic rocks such as limestone come from animal remains and chemical precipitation; coal comes from plant remains.

Metamorphic Rocks

Metamorphic rocks literally mean rocks that have changed form (table 4). This change can be caused by heat and pressure that occur below the surface of the Earth or by heat from upward-moving hot magma or melted rocks. Chemical reactions can also change rock form. Metamorphic rocks are found in Illinois as boulders and pebbles in the glacial drift. Foliated metamorphic rocks contain minerals that occur in layers; unfoliated rocks are massive.