DISTRIBUTION OF ELEMENTS IN THE EARTH’S CRUST
By far the most abundant element in the earth’s crust is oxygen, which accounts for nearly half its total weight. Silicon, the next most abundant, makes up nearly a quarter. These are followed in quantitative order by aluminium, iron, calcium, sodium, potassium, magnesium and titanium. These nine elements together make up practically the entire earth’s crust, while all the remaining elements form just under one per cent of its weight.
We have only to consider the distribution of elements to realize that silicates and quartz are by far the most common minerals of the earth’s crust. The amount of any element in the crust is not, however, always directly related to the abundance of the minerals in whose composition it forms a significant part. There are, for instance, some relatively abundant elements which do not form part of any known mineral. Rubidium, for example, is the seventeenth most important element, yet the only mineral of which it forms an appreciable part is the rare pegmatite mineral, rhodozite. Hafnium, which is far more common than, say, antimony or bismuth, forms no separate mineral at all. On the other hand, silver, which forms only o.ooooi per cent by weight of the crust, and is amongst the least abundant of elements, is present in a considerable number of minerals. Another relatively rare element is lead, yet it forms the very common mineral galena. The ability or inability of elements to form distinct minerals depends upon their chemical properties. Some elements, whose total amount within the crust is quite significant, are found only as traces or impurities in the minerals of other elements. Zircon, for instance, which is present as an accessory mineral in most igneous rocks, almost always contains some traces of hafnium and thorium. Some elements are thus more or less ‘hidden’ in minerals, and it is not surprising that for a long time they went unnoticed. Other elements again, whose total percentage in the crust is low, form a surprisingly large number of mineral species, owing to their chemical properties. The large number and great variety of antimony minerals, for instance, seem out of proportion to the relatively minor quantity of antimony present in the crust.