Substitutional isomorphism

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

Figure 3A. Schematic of a brucite sheet showing the relative locations of all Mg2+ by removing the top layer of oxygen. Minerals may contain any combination of Al3+ and Mg2+, while Fe3+ or Fe2+ may also substitute isomorphously (from Taylor and Ashcroft, 1972, with...

The lanthanides form a series of ions of closely related size and bonding characteristics and in many respects resemble Ca +, for which they often substitute isomorphously in biological systems. Since different Ln ions can be probed with particular spectroscopic techniques (e.g. Eu + and Tb +, fluorescence Gd +, ESR Nd +, electronic spectra), in favourable circumstances it should be possible to obtain information about the binding site of spectroscopically inactive Ca + in several ways. Systems smdied include the calciumbinding sites in calmodulin, trypsin, parvalbumin, and the Satellite tobacco necrosis virus. 

Other nuclei which can substitute isomorphously the usual framework elements in zeolitic materials are observable by solid-state NMR, e.g. Be, Ge and 69,71oa. Charge compensating cations, like Li, 23i ja, 39 or are suitable for NMR... 

Physical, chemical and mineralogical properties of the iron oxides can be variable. Aluminium (Al(III)) may substitute isomorphically for Fe(III) due to very similar ionic radii (Fe(III) 0.73 A Al(III) 0.61 A) and the same valence. Norrish and... 

Another aspect of the impurity doping which will be common in mineral particles is illustrated by the effect of Nb(V) on anatase. This ion substitutes isomorphical-ly for Ti(IV) and as an n dopant creates a Shottky barrier at the interface. This assists injection of an electron from a reducing solute into the conduction band (1J[). The flat band is also shifted cathodically. It has recently been claimed that the relative inefficiency of haematite as a photoanode is a function of low mobility of carriers and that this problem may be... 

Some mineral structures allow many chemical substitutions, or cation/anion exchanges, such as Mg replaced by Fe, Mn, Fe, or Af and replaced by OH", F , or Cl". Where the valency is the same (homovalent or isovalent exchange, as opposed to heterovalent), this is known as isomorphic substitution isomorphism for the phenomenon, but this term is also used in a different way concerning crystal faces isotypism or isostructuralism are sometimes used and the last is, by far, the most appropriate). 

Iron and manganese occur in a number of soil minerals. Sodium and chlorine (as chloride) occur naturally in soil and are transported as atmospheric particulate matter from marine sprays (see Chapter 10). Some of the other micronutrients and trace elements are found in primary (unweathered) minerals that occur in soil. Boron is substituted isomorphically for Si in some micas and is present in tourmaline, a mineral with the formula NaMg3AlgB3Sig027(0H,F)4. Copper is isomorphically substituted for other elements in feldspars, amphiboles, olivines, p5Toxenes, and micas it also occurs as trace levels of copper sulfides in silicate minerals. Molybdenum occurs as molybdenite (M0S2). Vanadium is isomorphically substituted for Fe or A1 in oxides, pyroxenes, amphiboles, and micas. Zinc is present as the result of isomorphic substitution for Mg, Fe, and Mn in oxides, amphiboles, olivines, and pyroxenes and as trace zinc sulfide in silicates. Other trace elements that occur as specific minerals, sulfide inclusions, or by isomorphic substitution for other elements in minerals are chromium, cobalt, arsenic, selenium, nickel, lead, and cadmium. 

Gatineau, L., 1964b. Structure reelle de la muscovite, repartition des substitution isomorphes. Doctoral thesis. L Universite de Paris. 

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