Minerals Found in Soil: Primary and Secondary

After reading this article you will learn about the primary and secondary minerals found in soil.

Primary Minerals:

1. Feldspars:

Feldspars belong to the group of minerals that are light in weight and are divided into orthoclase and plagioclase. In orthoclase, potassium is prominent. Plagioclase is divided into albite (high in sodium), anorthite (nigh in calcium), and oligoclase which consists of both albite and anorthite.

Plagioclase weathers more rapidly than orhoclase. Potash feldspars are present in granite and similar other igneous rocks while plagioclase feldspars form important constituents of basic igneous rocks like basalt, gabbro etc. In the soil, feldspars are usually present in the sand and silt fractions.

2. Quartz:

It is another mineral which is widespread. It is one of the most resistant minerals. Quartz is the silica minerals. Serpentine, a hydrous magnesium silicate, occurs more commonly as a secondary product. Garnets are characteristic of metamorphic rocks and are very hard and very resistant to weathering.

Quartz is an important constituent of all the three classes of rocks and is the dominant mineral in granite, sandstone, rhyolite etc. Among metamorphic rocks, gneiss and quartzite contain appreciable quantities of this mineral. It is present in sand and silt fractions. Even the clay fraction of some soils contains quartz.

3. Micas:

Micas occur both as primary minerals in igneous rocks and as secondary minerals in altered products of feldspars and other minerals. The two important types are muscovite (white mica) a potassium aluminium silicate and biotite (black mica) a potassium, aluminium, magnesium, iron silicate. Biotite weathers more rapidly than muscovite.

4. Hornblende’ and Actinolite:

These are the two important minerals of the amphibole group of silicate, are also known as ferro-magnesium minerals. These minerals contain a number of bases like calcium, magnesium, iron (ferric), manganese, titanium, sodium, potassium etc. Hornblende is more abundant in acid as well as basic igneous rocks. It is found in soil, especially in sand and silt fractions. The important minerals belonging to the pyroxene group are augite and eustatite. These minerals are usually predominant in basic igneous rocks like basalt, gabbro etc. They are rich in calcium, magnesium and iron.

Secondary Minerals:

Important secondary non-silicate minerals are calcite (CaCO₃), magnetite (MgCO₃) dolomite (CaCO₃. MgCO₃) and gypsum (CaSO₄. 2H₂O) (See Table 20.1).

Clay minerals are the most important secondary minerals. These have been synthesised from products of weathering, are colloidal in nature. Clay minerals are crystalline in nature and are very important constituents of soils. Most of the physical, chemical and morphological properties of soils are influenced by these clay minerals.

Clay minerals are hydrous aluminium silicate, frequently with some replacement of aluminium by iron and magnesium. The three most important groups of clay minerals are kaolinite (non-expanding type with two layers), montmorillonite (expanding type with three layers) and illite.