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50.3 MODELS OF OCCURRENCE OF METALS

Elements occur in nature in two states: native state and combined state.

  1. 1.     NATIVE STATE

The metals are said to be in native state if they are found in their elementary form. Generally, less active metals are found in native state. The common examples of metals which occur in native state are gold, silver, copper, platinum, etc.

  1. 2.     COMBINED STATE

The metals are said to occur in the combined state if they are found in nature in the form of their compounds. Generally, the reactive metals occur in the form of their compounds

In the combined state the metals are found in the crust of the earth as oxides, carbonates, sulphides, silicates, phosphates, etc. The naturally occurring chemical substances in the earth’s crust which can be obtained by mining are known as minerals. Minerals extracted from the crust of the earth are not pure but instead they are associated with large number of earthly, rocky and silicious impurities. The impurities associated with the minerals are collectively known as gangue or matrix

Every mineral of the metal cannot be used for its extraction. In some cases, economic factors while in others availability of the mineral may be hinderance. The mineral from which the metal can be economically and conveniently extracted is called ore. For example, earth’s crust contains aluminium in the form of two well-known minerals, bauxite (AI2O3.2H2O) and china clay (AI203.2SiO2.2H2O), but the extraction of aluminium is cheaper and easy from bauxite. Hence, ore of aluminium is bauxite. Similarly, minerals of copper are copper glance (Cu2S), cuprite (CU2O), malachite
(CuCO3. Cu(OH)2), copper pyrites (CuFeS2), etc., but the ore of copper is copper pyrites. Thus, it can be concluded that all the ores are minerals but all the minerals are not ores.

Examples of some important ores are given in Table 50.3.

Table 50.3. Metals and Their Ores

Metal Ore   Composition
Potassium Sylvine KCI
Sodium Rock-salt NaCI
Calcium Limestone CaCO3
Gypsum CaSO4, 2H20
Magnesium Magnesite MgCO3
Dolomite MgCO3, CaCO3
Aluminium Bauxite AI2O3, 2H2O
Diaspore AL2O3H2O
Kaolinite AL2(OH)4 . Si2O5
(a form of clay)
Titanium Rutile TiO2
Ilmenite FeTiO3
Zinc Zinc blende blende ZnS
or Sphalerite
Calamine ZnCO3
Zincite ZnO
Iron Haematite Fe2O3, xH2O
Iron pyrite FeS2
(Fool’s gold)  
Magnetite Fe3O4
Tin Cassiterite or SnO2
Tin stone
Chromium Chromite FeCr2O4
Manganese Pyrolusite MnO2
Lead Galena PbS.
Copper Chalcopyrite CuFeS2
or Copper pyrites
Malachite CuCO3, Cu(OH)2
Cuprite Cu2O
Copper glance Cu2S
Mercury Cinnabar HgS
Silver Argentite Ag2S
Gold Native metal Au

 It may be noted that the metals at top tend to occur more commonly as chlorides, carbonates and sulphates. Metals in the middle commonly occur as oxides and those which are
lower in the series occur as sulphides. The metals at the bottom such as silver, platinum and gold exist in their native state.

A few metals in the decreasing order of their abundance in the earth’s crust are listed as follows:

Aluminium, Iron, Calcium, Sodium,

Potassium, Magnesium, Titanium

 

DECREASING ABUNDANCE IN EARTH’S CRUST

SOLVED EXAMPLES

 Example 50.1 What type of metals are likely to exist in native state in nature? Give some examples of such metals.

Solution. The less reactive metals are likely to exist in nature state. These metals lie below hydrogen in the activity series. Some examples are gold, silver, platinum, copper, etc.

Example 50.2 Differentiate between “minerals” and “ores”.

Solution. Minerals are the naturally occuring chemical substances in the earth’s crust which can be extracted by mining. The minerals from which a metal can be extracted economically and conveniently
are. called ores. For example, minerals of copper are copper glance (Cu2S), cuprite (Cu20) and copper pyrites (CuFeS2) but its ore is copper pyrites. All ores are minerals but all minerals are not ores.

Example 50.3 Predict the modes of occurrence of the following three types of metals:

(i) Highly reactive (such as sodium).

(ii) Moderately reactive (such as iron).

 (iii) Noble metal (such as gold, platinum).

Solution.(i) Highly reactive metals occur in combined stare generally as their chlorides, sulphates or carbonates.

(ii) Moderately reactive metals generally occur as their oxides.

(iii) Noble metals exist in free state or native state.