There are many kinds of minerals, but over 95% of the earth’s crust is igneous rock, and the igneous rocks contain not more than a few minerals and most of these are part of just four groups or families: these are called the families or groups of igneous rock-forming minerals. The light-colored minerals are silica-rich and less dense. Those are called the felsic minerals. The darker mafic minerals contain iron aw sell as magnesium, are denser, and only melt at high temperatures. The rock forming minerals can be identified by observing some basic physical properties. The soil is formed by the weathering of the rocks and minerals.
The most common family of minerals, the feldspars come in a variety of colors, including pink, white or gray. The most distinctive property is the tendency of the mineral to split or cleave in two directions. The pink variety of feldspar is called orthoclase or potassium feldspar. All the feldspars can scratch glass. Chemically, the feldspars are aluminum silicates with varying amounts of calcium (Ca), sodium (Na) and potassium (K).
Quartz can be easily distinguished by great hardness, clear or light color and unlike the feldspars, it does not have any cleavage. Instead, it fractures into smooth rounded surfaces much like glass does (that is called conchoidal fracture). When crystals are present, they have a distinctive hexagonal shape. Quartz is a simple silicate composed only of silicon (Si) and oxygen (O).
A mineral familiar to most people, mica’s most distinctive property is the tendency to cleave into very thin sheets. This family has 2 varieties. Biotite is a dark variety of mica, while muscovite is a lighter variety. The micas are softer than glass. Both micas are complex silicates with varying amounts of aluminum (Al), potassium (K) and iron (Fe).
The minerals contain Fe and Mg in their chemical structure. The minerals are characterized by hardness, dark color (except for the bright green of olivine). The density of the ferromagnesian is greater. Amphibole (hornblende) is black and has a poorly developed cleavage. Pyroxene (augite) is usually greenish-black, also with cleavage. Actually, olivine has no cleavage, occurs in granular masses, and has a bright apple-green glassy appearance. It is the main constituent of the Earth’s mantle, the very thick layer underlying the thin continental or oceanic crust. It is also known to many as the gemstone peridot.
Md Nayem Hasan Munna
Soil and Environmental Sciences
University of Barisal, Bangladesh