Azurite's formula: Cu
3
(CO
3
)
2
(OH)
2
Malachite's formula: Cu
2
(CO
3
)(OH)
2 cuprite

Chemical Formula: Cu 2 O, Copper Oxide

Class: Oxides and Hydroxides
Green malachite is closely associated with azurite in many ways. Not only do they frequently occur together (pictured above), they also have very similar formulae.
Malachite can also replace azurite, making a pseudomorph, or an exact copy of an azurite crystal (only now instead of being blue, it would be green). Compare their formulas:
Azurite's formula: Cu
3
(CO
3
)
2
(OH)
2
Malachite's formula: Cu
2
(CO
3
)(OH)
2
The charges on the copper ions are the same for both minerals at positive two (each hydroxide has a charge of negative one and each carbonate has a charge of negative two).
But what causes the color change from azurite to malachite if the charge on the copper remains the same? Consider the formulas if they are rewritten as so:
Azurite's formula: Cu(OH)
2
-2(CuCO
3
)
Malachite's formula: Cu(OH)
2
-CuCO
3
Notice the different amounts of CuCO
3
in the two formulas. The azurite seems to have an extra CuCO
3
, but the transformation could be explained by an addition of an extra
Cu(OH)
2
to azurite's formula to make two malachites as in the following equation:
Cu(OH)
2
-2(CuCO
3
) + Cu(OH)
2
----> 2{Cu(OH)
2
-CuCO
3
}
{AZURITE} 2{MALACHITE}
Since the Cu(OH)
2
is more oxidized than the CuCO
3
, the malachite is therefore more oxidized than azurite. This means that malachite represents a later stage of oxidation and the increased oxidation is what causes the color change. The actual formula for the conversion is a bit more involved and includes the addition of a water molecule to two azurite molecules and the release of a carbon dioxide molecule; leaving behind three malachite molecules. The equation is represented as follows:
2{Cu(OH)
2
-2(CuCO
3
)} + H
2
O ---------> 3{Cu(OH)
2
-CuCO
3
} + CO
2
2{AZURITE} {WATER} 3{MALACHITE} {CARBON DIOXIDE}

 Chemistry: CuSiO 3 - nH 2 O, Hydrated copper silicate