Haematite impurity

Cadmia, it will be remembered, is the impure zinc oxide, containing sometimes lead and copper oxides, from the furnaces in which brass was smelted. Misy was the partly oxidized iron or copper pyrites, essentially basic sulphates of iron and copper. Synopian red was haematite. This mixture, assuming the reducing action of the fuel in the furnace, or of any other reducing agent not specified in the recipe would yield an alloy of gold and zinc, with some copper and perhaps some lead. 

Beneath the brown haematite ores of Northamptonshire, an impure unaltered ferrous carbonate deposit occurs which is bluish or greenish-grey in appearance. The depth at which it lies represents the depth to which weathering or oxidation of the upper layers has occurred. 

Selective adsorption may also have dramatic consequences for crystal growth if certain crystal planes can be blocked ). Trace amounts of Impurities may in this way determine the eventual crysted shape. This feature plays an Important role in the preparation of (tailor-made) colloids (Volume IV) obviously adsorption kinetics plays an Important role here. Certain substances may of course work the other way around In that they "attack the solid. Examples are some sulfur-containing compounds with silver halides, and chelating agents with haematite (a-Fe O ). 

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... 

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