The Earth Contents

4. Rock Cycle

Ionic bonding varies between different rock minerals

Many rock minerals contain bonds which are intermediate between ionic and covalent character, and elements so bonded are dissolved by water only reluctantly. The percentage ionic character of the bonds between the most common rock-forming elements and oxygen varies between 40% to 90%. Both alkali elements and alkaline Earth elements have a high degree of ionic character which allows them to be readily solubilised by water. Their behaviour contrasts with aluminium and silicon, which have bond characters that are intermediate between ionic and covalent. These atom scale properties exert an important control on the separation of elements during weathering, transport and deposition. Hence calcium, magnesium, sodium and potassium nearly always predominate over aluminium and silicon in natural waters derived by weathering silicate rocks, even though these rocks consist of 5 to 10 times more aluminium and silicon than calcium, magnesium, sodium and potassium.

Element % Ionic character
Aluminium 60
Calcium 79
Iron (II) 54
Iron (III) 69
Magnesium 71
Potassium 87
Silicon 48
Sodium 83

Why does calcium, magnesium, sodium and potassium nearly always predominate over aluminium and silicon in natural waters derived by weathering silicate rocks, even though these rocks consist of 5-10 times more aluminium and silicon than these other elements?