cation exchange

…between titanium and other C-type cations. Aluminum can partially substitute for silicon in the tetrahedral (T) site. Partial substitution of fluorine (F), chlorine, and oxygen for hydroxyl (OH) in the hydroxyl site is also common. The complexity of the amphibole formula has given rise to numerous mineral names within the…

…an extensive range of possible cation substitutions, amphiboles crystallize in both igneous and metamorphic rocks with a broad range of bulk chemical compositions. Because of their relative instability to chemical weathering at the Earth’s surface, amphiboles make up only a minor constituent in most sedimentary rocks.

…by the substitution of trivalent cations (Al³⁺, Fe³⁺, etc.) for divalent cations (Mg²⁺, Fe²⁺, etc.). Chlorites with a muscovite-like silicate layer and an aluminum hydroxide sheet are called donbassite and have the ideal formula of Al_4.33(Si₃Al)O₁₀(OH)₈ as an end-member for the dioctahedral chlorite. In many cases, the octahedral aluminum ions…

Common cations that coordinate the octahedral sheets are Al, Mg, Fe³⁺, and Fe²⁺; occasionally Li, V, Cr, Mn, Ni, Cu, and Zn substitute in considerable amounts. If divalent cations (M²⁺) are in the octahedral sheets, the composition is M2+/3 (OH)₂O₄ and all the octahedrons…

…are able to adsorb certain cations and anions and retain them around the outside of the structural unit in an exchangeable state, generally without affecting the basic silicate structure. These adsorbed ions are easily exchanged by other ions. The exchange reaction differs from simple sorption because it has a quantitative…

…and the process is called cation exchange. Those having fixed positive charges correspondingly exchange negative charges, or anions, and are said to undergo anion exchange.

Other cations known to substitute—albeit in only relatively minor amounts—within the dolomite structure are barium and lead for calcium and zinc and cobalt for magnesium.

The cation-exchange capacity, or the ability of a charged metal atom in the structure to exchange places with another in aqueous solution, is low. Once formed, kaolisols may subsequently become the parent material for new soils.

…the sheets are cross-linked with cations—for example, aluminum in muscovite—and hydroxyl pairs complete the coordination of these cations (see figure). Thus, the cross-linked double layer is bound firmly, has the bases of silica tetrahedrons on both of its outer sides, and has a negative charge. The charge is balanced by…

, Na⁺, a sodium cation) by a divalent ion (e.g., Ca²⁺, a calcium cation) requires further substitutions to keep the structure electrically neutral.

…the charge of the substituting cations. The Xcation sites in general are larger than the Ycation sites. Extensive atomic substitution occurs between the ideal end-member compositions. Most pyroxenes have only limited substitution of aluminum for silicon in the Z(tetrahedral) site. When a substituting ion differs in charge, electrical neutrality is…

The M1 cation strip is bonded to oxygen atoms of two oppositely pointing tetrahedral chains. Together, these form a tetrahedral-octahedral-tetrahedral (t-o-t) strip. A schematic projection of the pyroxene structure perpendicular to the caxis and the relationship of the pyroxene cleavage to the t-o-t strips or I beams…

…M⁺ is the interlayer exchangeable cation expressed as a monovalent cation and where x and y are the amounts of tetrahedral and octahedral substitutions, respectively (0.2 ≤ x + y ≤ 0.6). The smectites with y > x are called montmorillonite and those with x > y are known as…

Substitutions of aluminum cations (Al³⁺) for silicon cations (Si⁴⁺) constitute the chief imbalance, but the net charge deficiency may be partially balanced by other substitutions within the mica layer; there is always a residual net charge deficiency commonly in the range from 0.6 to 0.8 per O₁₀(OH)₂. This…

…framework allows reversible dehydration and cation exchange, properties which vary considerably with chemical and structural differences. Dehydration character varies with the way water is bound in the structure. For those zeolites in which water is tightly bound, dehydration occurs at relatively high temperatures; by contrast, in certain zeolites with large…