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- mineral deposit
- sulfide mineral
- asbestos
- sulfate mineral
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- National Institute of Aging - Vitamins and Minerals for Older Adults (Mar. 21, 2025)
Polymorphism is the ability of a specific chemical composition to crystallize in more than one form. This generally occurs as a response to changes in temperature or pressure or both. The different structures of such a chemical substance are called polymorphic forms, or polymorphs. For example, the element carbon (C) occurs in nature in two different polymorphic forms, depending on the external (pressure and temperature) conditions. These forms are graphite, with a hexagonal structure, and diamond, with an isometric structure. The composition FeS2 occurs most commonly as pyrite, with an isometric structure, but it is also found as marcasite, which has an orthorhombic internal arrangement. The composition SiO2 is found in a large number of polymorphs, among them quartz, tridymite, cristobalite, coesite, and stishovite. The stability field (conditions under which a mineral is stable) of these SiO2 polymorphs can be expressed in a stability diagram, with the external parameters of temperature and pressure as the two axes. In the general quartz field, there is additional polymorphism leading to the notation of high quartz and low quartz, each form having a slightly different internal structure. Cristobalite and tridymite are the high-temperature forms of SiO2, and indeed these SiO2 polymorphs occur in high-temperature lava flows. The high-pressure forms of SiO2 are coesite and stishovite, and these can be found in meteorite craters, formed as a result of high explosive pressures upon quartz-rich sandstones, and in very deep-seated rock formations, as from Earth’s upper mantle or very deep in subduction zones.
Chemical composition
The chemical composition of a mineral is of fundamental importance because its properties greatly depend on it. Such properties, however, are determined not only by the chemical composition but also by the geometry of the constituent atoms and ions and by the nature of the electrical forces that bind them. Thus, for a complete understanding of minerals, their internal structure, chemistry, and bond types must be considered.
Various analytical techniques may be employed to obtain the chemical composition of a mineral. Quantitative chemical analyses mainly use so-called wet analytical methods (e.g., dissolution in acid, flame tests, and other classic techniques of bench chemistry that rely on observation), in which the mineral sample is first dissolved. Various compounds are then precipitated from the solution, which are weighed to obtain a gravimetric analysis. A number of analytical procedures have been introduced that provide faster but somewhat less accurate results. Most analyses use instrumental methods such as optical emission, X-ray fluorescence, atomic absorption spectroscopy, and electron microprobe analysis. Relatively well-established error ranges have been documented for these methods, and samples must be prepared in a specific manner for each technique. A distinct advantage of wet analytical procedures is that they make it possible to determine quantitatively the oxidation states of positively charged atoms, called cations (e.g., Fe2+ versus Fe3+), and to ascertain the amount of water in hydrous minerals. It is more difficult to provide this type of information with instrumental techniques.
To ensure an accurate chemical analysis, the selected sample, which might include several minerals, is often made into a thin section (a section of rock less than 1 mm thick cemented for study between clear glass plates). To reduce the effect of the impurities, an instrumental technique, such as electron microprobe analysis, is commonly employed. In this method, quantitative analysis in situ may be performed on mineral grains only 1 micrometre (10−4 centimetre) in diameter.
Mineral formulas
Elements may exist in the native (uncombined) state, in which case their formulas are simply their chemical symbols: gold (Au), carbon (C) in its polymorphic form of diamond, and sulfur (S) are common examples. Most minerals, however, occur as compounds consisting of two or more elements; their formulas are obtained from quantitative chemical analyses and indicate the relative proportions of the constituent elements. The formula of sphalerite, ZnS, reflects a one-to-one ratio between atoms of zinc and those of sulfur. In bornite (Cu5FeS4), there are five atoms of copper (Cu), one atom of iron (Fe), and four atoms of sulfur. There exist relatively few minerals with constant composition; notable examples include quartz (SiO2) and kyanite (Al2SiO5). Minerals of this sort are termed pure substances. Most minerals display considerable variation in the ions that occupy specific atomic sites within their structure. For example, the iron content of rhodochrosite (MnCO3) may vary over a wide range. As ferrous iron (Fe2+) substitutes for manganese cations (Mn2+) in the rhodochrosite structure, the formula for the mineral might be given in more general terms—namely, (Mn, Fe)CO3. The amounts of manganese and iron are variable, but the ratio of the cation to the negatively charged anionic group remains fixed at one Mn2+or Fe2+ atom to one CO3 group.
Compositional variation
As stated above, most minerals exhibit a considerable range in chemical composition. Such variation results from the replacement of one ion or ionic group by another in a particular structure. This phenomenon is termed ionic substitution, or solid solution. Three types of solid solution are possible, and these may be described in terms of their corresponding mechanisms—namely, substitutional, interstitial, and omission.
Substitutional solid solution is the most common variety. For example, as described above, in the carbonate mineral rhodochrosite (MnCO3), Fe2+ may substitute for Mn2+ in its atomic site in the structure.
The degree of substitution may be influenced by various factors, with the size of the ion being the most important. Ions of two different elements can freely replace one another only if their ionic radii differ by approximately 15 percent or less. Limited substitution can occur if the radii differ by 15 to 30 percent, and a difference of more than 30 percent makes substitution unlikely. These limits, calculated from empirical data, are only approximate.
The temperature at which crystals grow also plays a significant role in determining the extent of ionic substitution. The higher the temperature, the more extensive is the thermal disorder in the crystal structure and the less exacting are the spatial requirements. As a result, ionic substitution that could not have occurred in crystals grown at low temperatures may be present in those grown at higher ones. The high-temperature form of KAlSi3O8 (sanidine), for example, can accommodate more sodium (Na) in place of potassium (K) than can microcline, its low-temperature counterpart.
An additional factor affecting ionic substitution is the maintenance of a balance between the positive and negative charges in the structure. Replacement of a monovalent ion (e.g., Na+, a sodium cation) by a divalent ion (e.g., Ca2+, a calcium cation) requires further substitutions to keep the structure electrically neutral.
Simple cationic or anionic substitutions are the most basic types of substitutional solid solution. A simple cationic substitution can be represented in a compound of the general form A+X− in which cation B+ replaces in part or in total cation A+. Both cations in this example have the same valence (+1), as in the substitution of K+ (potassium ions) for Na+ (sodium ions) in the NaCl (sodium chloride) structure. Similarly, the substitution of anion X− by Y− in an A+X− compound represents a simple anionic substitution; this is exemplified by the replacement of Cl− (chlorine ions) with Br− (bromine ions) in the structure of KCl (potassium chloride). A complete solid-solution series involves the substitution in one or more atomic sites of one element for another that ranges over all possible compositions and is defined in terms of two end-members. For example, the two end-members of olivine [(Mg, Fe)2SiO4], forsterite (Mg2SiO4) and fayalite (Fe2SiO4), define a complete solid-solution series (called the forsterite-fayalite series) in which magnesium cations (Mg2+) are replaced partially or totally by Fe2+.
In some instances, a cation B3+ may replace some A2+ of compound A2+X2−. So that the compound will remain neutral, an equal amount of A2+ must concurrently be replaced by a third cation, C+. This is given in equation form as 2A2+ ←→ B3++ C+; the positive charge on each side is the same. Substitutions such as this are termed coupled substitutions. The plagioclase feldspar series exhibits complete solid solution, in the form of coupled substitutions, between its two end-members, albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Every atomic substitution of Na+ by Ca2+ is accompanied by the replacement of a silicon cation (Si4+) by an aluminum cation (Al3+), thereby maintaining electrical neutrality: Na+ + Si4+ ←→ Ca2+ + Al3+.
The second major type of ionic substitution is interstitial solid solution, or interstitial substitution. It takes place when atoms, ions, or molecules fill the interstices (voids) found between the atoms, ions, or ionic groups of a crystal structure. The interstices may take the form of channel-like cavities in certain crystals, such as the ring silicate beryl (Be3Al2Si6O18). Potassium, rubidium (Rb), cesium (Cs), and water, as well as helium (He), are some of the large ions and gases found in the tubular voids of beryl.
The least common type of solid solution is omission solid solution, in which a crystal contains one or more atomic sites that are not completely filled. The best-known example is exhibited by pyrrhotite (Fe1 − xS). In this mineral, each iron atom is surrounded by six neighbouring sulfur atoms. If every iron site in pyrrhotite were occupied by ferrous iron, its formula would be FeS. There are, however, varying percentages of vacancy in the iron site, so that the formula is given as Fe6S7 through Fe11S12, the latter being very near to pure FeS. The formula for pyrrhotite is normally written as Fe1 − xS, with x ranging from 0 to 0.2. It is one of the minerals referred to as a defect structure, because it has a structural site that is not completely occupied.
