Short notes on the geochemical classification of elements



To consider the chemical composition and crystalline structure of minerals, we must understand the nature of chemical elements-the fundamental components of matter. An element cannot be broken into simpler particles by ordinary chemical processes.

Most common minerals consist of a small number- usually two to five-of different chemical elements. A total of 88 elements occur naturally in the Earth's crust. However, eight elements-oxygen, silicon, aluminum, iron, calcium, magnesium, potassium, and sodium-make up more than 98 per cent of the crust.

Each element is represented by a one- or two-letter symbol, such as O for oxygen and Si for silicon. The table shows a total of 108 elements, not 88, because 20 elements are produced in nuclear reactors but do not occur naturally.

An atom is the basic unit of an element. An atom is tiny; the diameter of the average atom is about 1/10,000,000,000 meters. A single copper penny contains about 1.56 1022 (1.56 followed by 22 zeros) copper atoms. An atom consists of a small, dense, positively charged center called a nucleus surrounded by negatively charged electrons.

An electron is a fundamental particle; it is not made up of smaller components. An electron orbits the nucleus, but not in a clearly defined path like that of the Earth around the Sun.

Most minerals are compounds. When ions bond together to form a mineral, they do so in proportions so that the total number of negative charges exactly balances the total number of positive charges. Thus, minerals are always electrically neutral. For example, the mineral quartz proportionally consists of one (4+) silicon cation and two (2- ) oxygen anions.

The 88 elements that occur naturally in the Earth's crust can combine in many ways to form many different minerals. In fact, about 3500 minerals are known.

However, the eight abundant elements commonly combine in only a few ways. As a result, only nine rock- forming minerals (or mineral "groups") make up most rocks of the Earth's crust. They are olivine, pyroxene, amphibole, mica, the clay minerals, quartz, feldspar, calcite, and dolomite.