Rocks are made up of minerals.
Depending on their origin, rocks are classified into three groups: igneous, metamorphic and sedimentary. Sedimentary rocks form through slow deposition, burial and compression of sediments at Earth's surface; metamorphic form through compression and high heat deep underground; and igneous rocks are formed when molten rock or magma worms its way up through the crust and solidifies -- often, though not necessarily, as part of a volcanic eruption. Rocks in each category are composed of minerals, each of which has unique texture and composition. There are thousands of different minerals known to science. The following are some of the most important groups.
Silicates
Rocks are made up of minerals, each of which has its own chemical composition. The most common class of minerals are the silicates, which contain oxygen and silicon -- the two most abundant elements in the Earth's crust. Silicon is the same element used to make the microchips in your computer. Depending on the mineral, silicates may also contain other elements like metals in addition to silicon and oxygen. As a general rule, they are never completely opaque and are usually relatively light. All have crystal structures based on a tetrahedral arrangement of atoms. Common examples include olivine, quartz, beryl, biotite and hornblende.
Oxides
Oxide minerals contain oxygen, water or hydroxyl (OH) coupled with one or more metals. They exhibit greater variation in terms of their physical properties than any other group of minerals, so they are difficult to define as a category except by composition. Common examples include corundum, which has the chemical formula Al2O3; cuprite, which has the chemical formula Cu2O; and hematite, which has the chemical formula Fe2O3.
Sulfides and Sulfates
Sulfides contain a metal coupled with the element sulfur. These minerals often have a metallic luster or shine to them, and many of them are valuable ores, since the metals they contain can be extracted from them. Mercury sulfide or cinnabar and pyrite or fool's gold are two common examples. Sulfates also contain sulfur, but as part of the sulfate ion (SO4 -2). All sulfate minerals are soft and either transparent or translucent. Calcium sulfate or gypsum is probably the most familiar example.
Carbonates
Carbonates are formed from a metal combined with the carbonate ion, CO3 -2. All minerals in this class are soft, brittle and vulnerable to attack by acid; carbonates will bubble and dissolve when submerged in warm hydrochloric acid, for example, much like baking soda combined with vinegar. Calcium carbonate or calcite is the most well-known example and is found in limestone. Dolomite (calcium magnesium carbonate) is another example of this class. Malachite contains copper and hydroxide in addition to the carbonate.
Categories
Rocks are mixtures or aggregates of minerals, and the composition of each class of rocks can vary widely depending on the kind of minerals present. Igneous rocks are rich in silicates; granites, for example, are rich in quartz and feldspar. Limestone is a sedimentary rock formed from calcite. Sedimentary deposits called evaporites can be left behind when lake or sea water evaporates -- gypsum or salt deposits frequently result. Gneiss is a metamorphic rock containing quartz, feldspars, micas and other minerals, while marble forms from limestone (calcite) or dolomite. The boundaries between many types of rocks are not well-defined since the precise mineral content may vary.
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