Minerals and rocks
Minerals must be distinguished from rocks. A mineral is a chemical compound with a given composition and a defined crystal 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. Over half of the mineral species known are so rare that they have only been found in a handful of samples, and many are known from only one or two small grains.
Physical properties of minerals
There are just over 4,000 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).
Chemical properties of minerals
Minerals may be classified according to their chemical makeup and are listed below in approximate order of their abundance in the Earth's crust.
Silicates
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.
Carbonates
The carbonates consist of those minerals containing the anion (CO3)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
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), celestine (strontium sulfate) and gypsum (hydrated calcium sulfate).
Halides
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
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), spinel (magnesium aluminium oxide - a common component of the mantle) and ice (hydrogen oxide!).
Sulfides
Many sulfides are also economically important as metal ores. Common sulfides include chalcopyrite (copper iron sulfide) and galena (lead sulfide).
Phosphates
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.
Elements
Elements group includes metals and intermetallic elements (gold, silver, copper), semi-metals and non-metals (antimony, bismuth, graphite, sulfur). This group also includes natural alloys, such as electrum (a natural alloy of gold and silver), phosphides, silicides, nitrides and carbides (which are usually only found naturally in a few rare meteorites).
Dietary mineral
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.
See also
External links
References
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