Sodium hydroxide white crust

The white crust often seen on pellets of sodium hydroxide is a mixture of sodium carbonate formed in this way and of sodium hydrogen carbonate formed in a similar reaction ... 

The metal, when freshly cut, has a silver-white surface, which rapidly becomes dull on exposure to air and becomes covered with crusts of sodium oxide, sodium hydroxide, and sodium carbonate. At ordinary temperatures, sodium has the consistency of wax, but at low temperatures it is brittle. 

Abundant yellow or white salt crusts are present on waste rock and at the surface of the soil. The crusts comprise alum-like sulfate minerals containing variable amounts of sodium, potassium, iron and aluminium, such as the mineral jarosite. They are often very soluble in water, releasing acid and precipitating ferric hydroxides. 

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