Mineral
Minerals are natural compounds formed through
geological processes. The term "mineral" encompasses not only the material's
chemical composition but also the
Mineral structures. Minerals range in composition from elements and simple salts to very complex
silicates (with most organic compounds usually excluded), with thousands of known forms. The study of minerals is called
mineralogy.
A mineral structure refers to the orderly spatial arrangement of atoms in its molecular structure. There are 14 basic arrangements of atoms in three dimensions, and all mineral structure recognised till now fit in one of these 14 arrangements.
Chemistry and structure define together a mineral. In fact, two or more minerals may have the same chemical composition, but different structures (these are known as polymorphs). For example, pyrite and marcassite[?] are both iron sulfide. Similarly, some minerals have different chemical compositions, but the same structure : for example, quartz (made from silicon and oxygen) and berlinite[?] (aluminium and phosphorous).
Structure influences greatly a mineral physical properties. For example, though diamond and graphite have the same composition, graphite is very soft, while diamond is the hardest of all minerals.
To be classified as a true mineral, a substance must be a solid and have a crystal structure. Mineral-like substances that don't strictly meet the definition are sometimes classified as mineraloids.
Minerals must be distinguished from rocks. A mineral is a chemical compound with a given composition and a defined atomic structure. A rock is a mixture of one or several minerals, in varying proportions.
The specific minerals in a rock vary a lot. Some minerals, like quartz, mica or talc are everywhere, while others have been found in only one or two locations worldwide.
There are about 3000 different mineral species known, and classifying them can range from simple (for a couple of hundred) to very difficult. A mineral can be identified by several physical properties, some of them being sufficient for full identification without equivocation. In other cases, minerals can only be classified by more complex analyses including some modern techniques such as chemical analysis and X-ray diffraction, which however, can be costly, time-consuming, and even risk damaging the sample.
Physical properties commonly used are :
- Hardness: the physical hardness of a mineral is usually measured according to the Mohs scale of mineral hardness.
- Luster indicates the way a mineral's surface interacts with light and can range from dull to glassy (vitreous).
- Color indicates the appearance of the mineral in reflected light (i.e. what it looks like to the naked eye).
- Cleavage describe the way a mineral may come apart or cleave in different ways. In thin sections, cleavage is visible as thin lines across a mineral.
- Fracture describes how a mineral breaks other than along natural cleavage planes.
- Specific gravity relates the mineral weight to the weight of an equal volume of water.
- Other properties: fluorescence (response to ultraviolet light), magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form).
Minerals may be classified according to their chemical makeup and are listed below in approximate order of their abundance in the Earth's
crust.
The largest group of minerals by far are the
silicates, which are composed largely of
silicon and
oxygen, with the addition of
ions such as
magnesium,
iron and
calcium. Some important rock-forming silicates include the
feldspars,
quartz,
olivines,
pyroxenes,
garnets and
micas.
The
carbonates consist of those minerals containing the
anion (CO
3)
2- and include
calcite and
aragonite (both calcium carbonate),
dolomite (magnesium/calcium carbonate) and
siderite[?] (iron carbonate). Carbonates are commonly deposited in marine settings when the shells of dead
planktonic life settle and accumulate on the sea floor. Carbonates are also found in
evaporitic settings (e.g. the
Great Salt Lake,
Utah) and also in
karst regions, where the dissolution and reprecipitation of carbonates leads to the formation of
caves,
stalactites and
stalagmites.
Sulfates all contain the sulfate cation, in the form
SO4. Sulfates commonly form in
evaporitic settings where highly saline waters slowly evaporate, allowing the formation of both sulfates and halides (q.v.) at the water-sediment interface. Common sulfates include
anhydrite[?] (calcium sulfate) and
gypsum (hydrated calcium sulfate).
The
halides are the group of minerals forming the natural
salts and include
fluoride, common salt (known as
halite[?]) and
sal ammoniac[?] (ammonium chloride). Halides, like sulfates, are commonly found in evaporitic settings such as
playa lakes[?] and landlocked seas (e.g. the
Red Sea).
Oxides are extremely important in
mining as they form the ores from which valuable metals can be extracted. They thus commonly occur as precipitates close to the Earth's surface. Common oxides include
haematite (iron oxide = rust),
spinel (magnesium aluminium oxide - a common component of the
mantle) and
ice (hydrogen oxide!).
Many
sulfides[?] are also economically important as metal ores. Common sulfides include
chalcopyrite (copper iron sulfide) and
galena (lead sulfide).
The
phosphate group actually includes any mineral with a tetrahedral unit
AO4 where A can be
phosphorus,
antimony,
arsenic or
vanadium. By far the most common phosphate is
apatite which is an important
biological mineral found in teeth and bones of many animals.
Dietary mineral[?] refers to inorganic compounds necessary for life and good nutrition.
Some of these are scientific minerals as salt; others are elements, as potassium, calcium, iron, zinc, magnesium, copper.
These can be naturally occurring in food or added in elemental or mineral form to food, as calcium carbonate, iron fillings, etc.
Some of these additives are from natural sources as ground oyster shells for calcium carbonate.
Sometimes minerals are added to the diet separately from food, as vitamin and mineral supplements and in dirt eating, called pica or geophagy.
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