Alums crystal structure

Cesium titanium alum forms violet octahedral crystals which are sparingly soluble in cold water, more soluble in hot. These crystals belong to the /3-type alum crystal structure, or those alums that contain a large unipositive ion. The compound is oxidized rapidly in air, and water solutions become turbid on exposure to air, precipitating titanic acid. 

Different compounds that crystallize with the same crystal structure, for example, the two alums, NaAl(SC>4)2 12H20 and NaFe(SC>4)2 12H20, are said to... 

Comparison of the crystal structures of (a) y alum NaAl(S04)2-12H20 and (b) p alum CsAl(S04)2 12H20. For clarity the bonds between the M1 atoms and the sulfate groups are represented by broken lines. 

It can still be seen in mineralogical museums how Berzelius classified many compounds as adducts of oxides, as when alum is written K20 A1203 4 S03 24 H20 whereas it was later written KA1(S04)2 12 H20 and the crystal structure indicates only half of the water molecule oxygen atoms directly connected to aluminum K[A1(0H2)6](S04)2(0H2)6. Actually, the large majority of minerals are mixed oxides (and though these formulae derive from the precursor ideas of electrovalent bonding, considering calcium sulfate as an adduct CaO S03, they have the undoubted... 

Attempts at identifying the influence of structme on stability have generally been inconclusive. For example, some alkali metal permanganates with comparable stmctures show similarities of decomposition behaviour [29], while, in contrast, the decompositions of several cobalt(lll) ammine azides show little evidence of structural influences [76], Significant differences in behaviour were found for the various crystal forms of the LiK tartrate hydrates [87] and, also, for the dehydrations of the isomorphic alums [20,43], However, some reactants, for example those prepared by the dehydration of hydrated metal carboxylates [5], may be amorphous to X-rays, thus preventing recognition of any control of stability by crystal structure. 

One possibility would be to introduce a concrete model (see E4.2) and to compare it to the original (see Fig. 4.4). This comparison leads clearly to the association, that there are tiny particles in an alum crystal which are arranged in a special type of chemical structure like spheres in the shown closest sphere packing original crystal and the concrete model exhibit the same octahedral form, the same smooth edges, smooth surfaces and uniform angles, one sphere correlates to one alum particle (see Fig. 4.4). 

Fig. 4.4 Alum crystals in saturated solution, alum crystal and simplified structural model...

Aluminum hydroxide precipitated freshly, for example, when filter alum is added to water in the water treatment process of coagulation, is more soluble than the thermodynamically-stable AKOHlajs) phase, gibbsite. This material has the same composition but a different, more well-defined crystal structure. The solubility product for gibbsite is... 

Anhydrous double sulfates ( alum anhydrides ) M M (S04)2 are formed by A1 and Ga with NH4 or the alkali metals Crystal structure determination establishes that Al is... 

Different compounds which crystallise with the same crystal stmcture, for example the two alums, NaAl(S04)2-12H20 and NaFe(S04)2.12H20, are said to be isomorphous or isostructural. As noted in Section 3.1.3, sometimes the crystal structure of a compound wiU change with temperature and with applied pressure. This is called polymorphism. Polymorphs of elements are known as aUotropes. Graphite and diamond are two allotropes of carbon, formed at different temperatures and pressures. 

On this principle, beautiful crystal baskets, ornaments, etc., can be built of alum, sulphate of copper, and bichromate of potash. The baskets are made of covered copper wire, and when the salts crystallize on them as a nucleus or centre, they are constantly removed to fresh solutions, in which they must be completely covered, until red, white, and blue sparkling crystal structures are formed. They wUl retain their brilliancy for an indefinite period by being put under a glass shade in which is placed a cup containing a little water. 

The rotation of the ammonium ion in salts at ordinary temperatures provides justification for the customary treatment of the ion as spherically symmetrical in the theoretical discussion of the structure of ionic crystals. Further, the rotation of molecules such as NHj and H20 about symmetry axes accounts for the fact that these molecules occupy positions in crystals with symmetry elements not compatible with those of the non-rotating molecule. Thus in Ni(NH3)6CU the NHj molecules lie on four-fold axes, and in alum the H2O molecules on three-fold axes. The rotation of the molecules,... 

Interesting crystals can also be made from supersaturated solutions of Epsom salts (MgS04 7 H20) and alum (KA1(S04)2 - 12 H2Oj, which is used for pickling and is available in the spice section of some grocery stores. Crystal shape directly relates to how the ions or molecules of a substance pack together. In fact, substances are often characterized by the shape of the crystals they form. Crystallography is the study of mineral crystals and their shapes and structure. 

Close study of the angles, indices, and axial ratios long since made it clear that every crystalline substance has a structure built upon a space -lattice" characteristic rtf the substance. It lias been established that this is due to the regular arrangement of the alums, molecules, or ions composing the substance. As shown by Table I. the lattice structures of crystals may be classified into 32 symmetry classes (point groups), which are further divided into seven systems. This topic also is discussed under Mineralogy. 

The internal structure of crystals of various alums has been studied by Bragg s X-ray method,1 and the conclusion arrived at is that the water of crystallisation is not to be regarded as in any way distinct from the other constituents of the molecule, since its removal necessitates the destruction of the crystalline structure. Werner s generic formula for the alums is in complete harmony with this, namely —... 

The labors of Pasteur and others have completely established the correlation which exists between molecular asymmetry and rotatory power. If the asymmetry exists only in the crystalline molecule, the crystal alone will be active if, on the contrary, it belongs to the chemical molecule the solution will show rotatory power, and often the crystal also if the structure of the crystal allows us to perceive it, as in the case of the sulphate of strychnine and the alum of amylamine. 

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