Aggregation iron compounds

The common gray, brown, or black flint result from admixed iron oxides or organic compounds. Pure quartz is usually colorless and transparent. The conchoidal fracture typical of flint was a boon to primitive humans, who desired clean, smooth, sharp edges that were not susceptible to uniform wear. This physical attribute and the densities of flint reflect the patterns of aggregation of its fibrous constituents. 

Increasing restrictions also apply to the chloride process, so that efforts are continually being made to use the iron chloride byproduct, e.g., in water treatment and as a flocculation agent [2.56], Another process for treating metal chorides with cement and alkaline compounds to produce rock-like aggregates for road building is described in [2.57]. 

To prepare colloid iron(III) hydroxide, heat 200 mL of distilled water in a beaker to 70°-90°C and leave an identical beaker of water at room temperature. Add 1 mL of 1 M FeCl3 to each beaker and stir. The warm solution turns brown-red in a few seconds, whereas the cold solution remains yellow (Color Plate 31). The yellow color is characteristic of low-molecular-mass Fe3+ compounds. The red color results from colloidal aggregates of Fe3+ ions held together by hydroxide, oxide, and some chloride ions. These particles have a molecular mass of 105 and a diameter of 10 nm, and they contain 103 atoms of Fe. 

A saturation transfer experiment demonstrated that the multiplicity of resonances arises from an equilibrium between different compounds macrocycles where a chloride ion is bound axially to iron and aggregates where the axial position is essentially free and the electronic 7t system gives rise to stacking interactions (see Sect. 4.3). 

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