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how rocks & minerals are classified

How Are Rocks and Minerals Classified? A Beginners Guide

Rocks, also known as stones, are natural substances that are solid aggregates of several minerals. For instance, granite, which is a type of rock, is made up of biotite, quartz and feldspar minerals. The lithosphere (the outer layer of the earth that is in solid form) is made of stone. Rocks have been utilized by human beings since time immemorial.

The minerals and metals contained in them have played a great part in the civilization of man. Rocks are made up of particles of minerals, which exist naturally in the earth and are formed from organic matter. Minerals in a rock are joined together by chemical bonds. The kinds and quantities of minerals in any rock are dependent on how it was formed.

A lot of rocks have silica (a compound of oxygen and silicon) that forms a huge portion of the earth's crust. Silica usually reacts with other compounds in rocks to form crystals. The amount of silica present in minerals and rocks is a huge factor when establishing their names and characteristics.

Geologists classify rocks according to properties like:

  • Permeability
  • Chemical composition
  • Mineralogy
  • Texture
  • Particles they contain
  • The size of the particles

These physical characteristics are caused by the processes that created the rocks. Over time, rocks can be altered from one form to another. The alteration leads to 3 major groups of rock:

  • Sedimentary rocks
  • Metamorphic rocks
  • Igneous rocks

The three groups are further subdivided into many classes. There are no clear boundaries between rocks that are similar in nature. By addition or deduction in the amounts of the minerals they have, they go through gradual changes one by one.The distinct properties of one type of rock might therefore be traced systematically and directly into those of another type.

Sedimentary Rocks

Sedimentary rocks occur at the surface of the earth from the amassion and cemention of fragments of pre-existing rocks, minerals and organic matter. They also occur as natural growths in water and precipitates of chemicalsthrough sedimentation.

Prior to being deposited, sediments are created by weathering and erosion of pre-existing rocks in a source site. Thereafter, agents of denudation, which are water, ice, wind, glaciers and mass movements, transport them to the deposit site.

Sedimentary rocks make up around 7.9 percent of the earth's crust. Shales form 82 percent of this, while limestone makes up 6 percent. Sedimentary rocks usually have fossils. These rocks are created under the force of gravity. They are normally deposited next to horizontal layers or strata and this gives them the name 'stratified rocks'.

Metamorphic Rocks

Metamorphic rocks occur when pre-existing rocks get subjected totemperature and pressure levels that are different from those in which the pre-existing rock was created. This process is referred to as metamorphism.

It results in great alteration of the physical and chemical characteristics of the rock. The pre-existing rocks may be sedimentary rocks, old metamorphic rocks or igneous rocks.

The pre-existing rock (referred to as the protolith)changes into other kinds of minerals or different forms of the original minerals, through the process of recrystallization. Temperature and pressure levels needed for this process tend to be higher than those in the surface of the earth.

Metamorphic rocks make up about 27.4 percent of the volume of the earth's crust. The 3 main groups of metamorphic rocks form their basis on the process of formation.

  • A transformation that is caused by heat is contact metamorphism. It occurs when magma intrudes and heats the surrounding rock.
  • A transformation that depends more on pressure than heat is pressure metamorphism. It happens when sediments are very deep beneath the earth's surface.
  • A transformation that depends on both the temperature and pressure is regional metamorphism. It is often seen in mountainous regions.

Metaphoric rocks are further divided into 2 general groups based upon their structure. Those rocks that have a texture are called foliated metamorphic rocks; the remaining ones are called non-foliated metamorphic rocks. The specific label of the stone is then established using the kinds of minerals it possesses.

Schists are foliated metamorphic rocks that mainly have lamellar minerals like micas. Gneisses have bands of varying brightness that can be viewed visually; an example is the granite gneiss. Other foliated rocks are;mylonite, slates and phyllites. Common examples of non-foliated rocks are serpentine, marble and soapstone.

Igneous Rocks

Igneous rocks are formed when magma or lava cools and solidifies. The magma may come from partial melts of original rocks in the earth's crust. Usually, the melting of stones arises due to: increase in heat, decrease in pressure, or an alteration in the composition.

Igneous rocks are classified into 2 major groups:

  • Intrusive/plutonic igneous rocks occur when magma cools and undergoes crystallization gradually in the earth's crust. Granite is an example of this type of rock.
  • Extrusive/volcanic igneous rocks occur when magma reaches the earth's surface as lava or as fragmental ejecta, creating minerals like basalt and pumice.

The chemical composition and the pace at which the magma cools produces a sequence referred to as 'Bowen's reaction series'. A lot of igneous rocks fall on this scale. Igneous rocks form around 64.7 percent of the volume of the earth's crust, which makes it the largest category.

Out this percentage, basalts and gabbros make up 66 percent, granite forms 16 percent, diorite and granodiorites take up 17 percent, syenites form 0.6 percent and peridotites make up 0.3 percent. The oceanic crust in itself is 99 percent basalt. The continental crust is formed majorly by granites and meta-granitoids.

Classification of Minerals

Mineral classification is based upon the chemical composition of the minerals. There exist at least 3000 different kinds of minerals which make it necessary to have a system to classify them.

The Dana system, which was created by Prof. James Dana from Yale University in 1848, is usually used in grouping minerals; though there are other grouping systems. This grouping system categorizes minerals into these major classes: carbonates, halides, mineraloids, native elements, oxides, phosphates, silicates, sulfates and sulfides.

  • Carbonates are a class of minerals that are composed of carbon, oxygen, together with a metallic element. Calcite better known as calcium carbonate is an example of a mineral of this type.
  • Halides are made up of halogen elements like bromine, chlorine, fluorine and iodine, together with metallic elements. Halides are super soft and are highly soluble in water. A common example under this category is halite which is known as sodium chloride.
  • Mineraloids are those minerals that do not fit perfectly into the other 8 classes of minerals. They include: amber, jet, mother of pearl and opal.
  • Native elements are a class of pure minerals. A lot of minerals exist in combinations of numerous chemical compounds. Native elements are those that occur naturally in a pure state, for example copper is a single element mineral.
  • Oxides arise when metals are combined with oxygen. This class consists of dull ores such as bauxite and gems such as sapphire and ruby.
  • Phosphates are created when there is break down of other minerals through weathering. They are usually bright in color and do not occur as much as the other groups of minerals.
  • Silicates form the largest family of minerals. They arise from the combination of metals with oxygen and silicon. There exist more silicates on earth than all the other minerals added together. Mica is a common example under this category.
  • Sulfates form from the combination of sulfur compounds with oxygen and metals. This is also a large family of minerals, which are soft and translucent in nature. Barite is a good example of a sulfate.
  • Sulfides are composed of sulfur compounds often together with a metal. They are heavy and brittle in nature. Pyrite is an example of a sulfide.

This mineral categorization assists in organizing numerous known minerals as well as to understand the formation of minerals. It is a wonder how thousands of minerals that can be categorized to fit at least 8 groups!

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