4.6: Other Important Rock Forming Minerals
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The final "Big Ten" mineral is calcite and was not previously discussed as it is not a silicate mineral. Calcite formation is most typically associated with sedimentary processes.
Calcite belongs to a family, or group of minerals, called the carbonates. The basic chemical and crystalline foundation for the carbonate group is the complex carbonate ion \(\ce{(CO3)^{-2}}\) where one carbon cation is surrounded by three oxygen anions in a triangular pattern.
Calcite forms by the ionic bonding of the complex carbonate ion with a calcium anion. The diagram above represents one unit cell of the mineral calcite.
Crystallization of Calcite
Calcite most commonly forms by:
1. biomineralization, where organisms like clams and corals extract calcium and carbonate ions from seawater or fresh water and then combine them inside their tissues to form their skeletal material
2. precipitation, where calcium and carbonate ions combine to precipitate micro-crystals of calcite from supersaturated seawater or circulating groundwater
3. evaporation, where inland lakes and epicontinental seas become isolated and slowly dry up leaving crystalline calcite plus other minerals
Dolomite is another common carbonate mineral closely related to calcite often occurring in similar environments of formation. Dolomite has many similar characteristics to calcite including its chemical formula \(\ce{CaMg(CO3)2}\).
Clay Minerals
Clay is both a size of material and also a whole group of silicate minerals that form through the processes involved with weathering of pre-existing silicate minerals in the presence of weak acid and water. As rain forms in the atmosphere, it combines with carbon dioxide to form a weak acid called carbonic acid with the chemical formula, \(\ce{H2CO3}\). This weak acid rains down on Earth’s surface and immediately attacks minerals exposed at the surface. The pre-existing minerals become altered in their chemical composition through a reaction called hydrolysis. Hydrolysis is a type of chemical weathering that forms a wide range of clay minerals.
Clay minerals are sheet silicates similar to micas. Their chemical formula is dependent upon the minerals from which they were derived however all clay minerals contain a great deal of aluminum and silica plus varying amounts of water. Individual clay crystals tend to be very small, and are important to the study of geology because they are the main components of fine-grained sediment (mud) and form a very common sedimentary rock, shale.
Other Rock-Forming Silicates
The silicate minerals that fall into the group of “other” are largely formed deep within the Earth’s crust where rocks are buried and experience intense changes in temperature and pressure during periods of mountain building (orogeny). Some of the pre-existing minerals change in physical form and chemical composition as they recrystallize in response to these altered conditions.
One common mineral formed this way is garnet. Garnets are largely formed through the alteration of clay minerals which are rich in aluminum and silica. These clay minerals would be found in the common sedimentary rock shale that is subjected to varying degrees of pressure and temperature changes during the collision of land and the formation of mountains.
| Mineral Groups | Common Rock Forming Mineral |
|---|---|
| Silicates | Olivine Pyroxene (Augite) Amphibole (Hornblende) Biotite Muscovite Calcium-rich plagioclase feldspar (anorthite) Sodium-rich plagioclase feldspar (albite) Potassium-rich Feldspar (orthoclase) Quartz Clay Garnet |
| Carbonates | Calcite, Dolomite |
| Oxides | Hematite, Magnetite |
| Halides | Halite |
| Sulfides | Pyrite, Chalcopyrite, Galena, Sphalerite |
| Sulphates | Gypsum |
| Phosphates | Apatite |
| Native Elements | Gold, silver, copper |
Oxides, Halides, and Sulfides
After carbonates, the next most common non-silicate minerals are the oxides, halides, and sulfides.
Oxides
Oxides consist of metal ions covalently bonded with oxygen. Iron oxides are important for producing iron ore deposits from which we extract metallic iron. The formation of Earth’s major iron ore deposits are important in studying Earth’s history. Prior to about two billion years ago (2 Ga), the early oceans were saturated with soluble iron due to the lack of free oxygen. So much iron existed in the water that our early oceans were thought to have been green in color from the dissolved iron. As oxygen levels slowly increased in the atmosphere and dissolved in the oceans, the oxygen immediately bonded with the iron forming the common ore minerals of hematite (\(\ce{Fe2O3}\)) and magnetite (\(\ce{Fe3O4}\)).
The most familiar oxide of iron is rust, which is a combination of iron oxides (\(\ce{Fe2O3}\)) and hydrated oxides. Hydrated oxides form when iron is exposed to oxygen and water. The red color that we often see in soil and rock is usually due to the presence of iron oxides. For example, the red sandstone cliffs in Zion National Park and throughout Southern Utah consist of white or colorless grains of quartz coated with iron oxide which serves as cement, holding the grains together.
Halides
The halide minerals contain the element chlorine ionically bonded with sodium or other cations. These minerals include halite (NaCl; table salt).
Halide minerals are important to the study of Earth’s history because their occurrence indicates a period of evaporation of sea water or other isolated bodies of water. A well-known example of modern halide mineral deposits created by evaporation is the Bonneville Salt Flats, located west of the Great Salt Lake in Utah.
Sulfides
Many important metal ores are sulfides, in which metals are bonded to sulfur. Significant examples include: pyrite (iron sulfide, sometimes called “fool’s gold”), chalcopyrite (iron-copper sulfide), galena (lead sulfide), and sphalerite (zinc sulfide). Sulfides are well known for being important ore minerals. For example, galena is the main source of lead, sphalerite is the main source of zinc, and chalcopyrite is the main copper ore mineral.
The iron sulfide mineral pyrite is of particular importance to geology. Pyrite is unstable in the presence of oxygen and will readily chemically decompose to form stable iron oxide. However, pyrite occurs in some of Earth’s early (older than 2.0 Ga) sedimentary rocks. Sediments are formed from the weathering of pre-existing rock exposed on Earth’s surface. Pyrite, existing as sedimentary clasts, or pieces, within these ancient rocks, indicates that surface conditions must have been lacking oxygen to allow the pyrite to remain unaltered and ultimately preserved.
Sulfate Minerals
Sulfate minerals contain a metal ion, such as calcium, bonded to a sulfate ion. The sulfate ion is a combination of sulfur and oxygen \(\ce{(SO4)^{-2}}\). The sulfate mineral gypsum \(\ce{(CaSO4 \cdot 2H2O)}\) is important to Earth’s history because it indicates formation from evaporating water and usually contains water molecules in its crystalline structure.
Phosphate Minerals
Phosphate minerals contain the complex cation \(\ce{(PO4)^{-2}}\) combined with various anions and cations. The best known and important phosphate mineral to the study of historical geology is apatite, \(\ce{Ca5(PO4)3(F,Cl,OH)}\). Variations of this mineral are found in teeth and bones.
Native Elements
Native element minerals, usually metals, occur in nature in a pure or nearly pure state. Gold, silver and copper are examples of common native element minerals. Carbon is often found as a native element in minerals such as graphite and diamond.
- calcite - the only "Big Ten" mineral that is not a silicate; composed of calcium, carbon, and oxygen; typically associated with sedimentary processes
- clay - a group of silicate minerals formed through the weathering of pre-existing silicate minerals; also a very small size of material
- halides - minerals consisting of chlorine bonded with sodium or other cations
- native elements - minerals that are composed of one element, existing in a pure or nearly pure state
- oxides - minerals consisting of metal ions covalently bonded with oxygen
- phosphates - minerals consisting of a \(\ce{(PO4)^{-2}}\) cation bonded with various ions
- sulfates - minerals consisting of metal ions bonded to a sulfate ion
- sulfides - minerals consisting of metal ions bonded with sulfur