Mineralogy. Isomorphism. Isomorphism types.

Isomorphism - property of atoms (or ions) one substances to exchange in structure atoms (or ions) others. The phenomena of isomorphism very widespread in minerals. So, the elemental composition of a mineral of wolframite is mapped by the formula (Fe, Mn) [WO4]. It represents an isomorphic intermixture where atoms of manganese replace atoms of iron and on the contrary, it is formal in structure can be expressed the formula nFe [WO4] X (100-n) X Mn [WO4]. Extreme terms of this number carry the name of ferberite Fe [WO4] and hubnerite Mn [WO4]. The mineral an olivine (Mg, Fe) 2 [SiO4] also represents an isomorphic intermixture where atoms of magnesium in structure are replaced with atoms of iron. Formally it can be expressed the formula nMg2 [SiO4] X (100-n) Fe2 [SiO4]. Terminating terms of this continuous number carry forsterite and fayalite names.

Along with simple cases there can be the difficult isomorphic displacement of the whole complexes in crystalline structures. A classical example of such difficult displacement are minerals from group of field spars - plagioclases. Plagioclases represent the continuous number of minerals where pair Ca2 + and Al3 + are replaced on pair Na + and Si4 + (CaAl«NaSi). Extreme terms of this number are termed as anorthite Ca [Al2Si2O8] and albite Na [AlSi3O8]. According to composition change physical properties of plagioclases, for example optical properties, density change also, etc.

Thus, two front view of isomorphism differs: more simple when the ions having identical valence are crossly replaced, - it is termed as the isovalence isomorphism, and difficult when there is a displacement of ions of different valences, the heterovalence isomorphism.

On a degree of perfection of isomorphic displacements it is possible to ooze two cases. In the first case displacement of one device by another can be in limits to 100 % is made, or complete, isomorphism. In the second case displacement can be partial from the 100-th shares, to several percent is imperfect, or restricted, isomorphism.

Many isomorphic impurities are not reflected by the mineral formula for their quantity is insignificant. So, in zinc blendes ZnS usually is present at a view of isomorphic impurity Fe, and sometimes Cd and B.

If there is an isomorphic displacement of one of elements (or complexes) others they are bracketed and separate from each other a comma, and the writing order depends on quantity of these of elements (or builders).

In what the isomorphism parent? V.M.Goldshmit has specified, that devices can replace each other in structures in that case when radiuses of ions (or atoms) are relatives and the odds does not exceed them 15 %. However, if quantities of radiuses of ions are close, it yet does not predetermine their isomorphic displacement. Concept about the ionic balls, as incompressible balls, truly only as a first approximation. The electric field stress, a charge and types of ions lead to such interaction between them when the correct conglobate shape of an ion is garbled, there is a polarisation (strain) of an electronic shell of an ion. Therefore, except affinity of the ionic radiuses, polarisation properties of ions too should be relatives. If the degree of polarisation of ions is various, even at very close radiuses of isomorphism between ions will not be (for example, there is no isomorphism between Na + and Cu +).

Polymorphism. This phenomenon is to a certain extent opposite to isomorphism and consists that identical substances on an elemental composition form various structures.

Devices and the difficult linkings can be the polymorphic. The origin of various polymorphic updatings (versions) is related to distinction in the conditions of their formation. Each of updatings has the structure, and from here and the specific properties.

Good example of polymorphism of carbon are minerals diamond and graphite. Their properties are absolutely various: diamond the firmest of minerals, graphite has hardness 1. Density of diamond 3.5, graphite 2.2. Diamond crystallises in a cubic syngony, graphite in the hexagonal. The parent in so various properties of the specified minerals speaks their structure, i.e. a location of atoms of carbon. Communication of atoms of carbon in graphite less strong, than in diamond, graphite structure leaf, in the form of flat hexagonal nets. The considerable distances between these nets also spot its properties: an easy splittability, smaller density, etc.

Some polymorphic updatings sulphur has also; naturally inconvertible is it is only grey a trimetric syngony.

Distinguish two views of polymorphism. The first view - enantiotropy - is characterised by convertibility (transition) of polymorphic updatings from one in another at certain temperatures and pressures. As an enantiotropy example quartz transitions in high-temperature version Sio2 - a tridymite, and also diamond transitions in graphite can serve. - a monotropy - one polymorphic updating (astable) can transfer the second view in another (stable), but return transition is impossible. A monotropy example is marcasite transition in pyrite.