'All Ores are Minerals but not all Minerals are Ores'. An Ore is a type
of Rock containing a Mineral from which Metals can be profitably extracted, or
in other words, an Ore contains a Mineral which contains sufficient high
concentration of a Metal, so as to enable profitable extraction of the desired
Metal. The main deciding factor in this context is the concentration of
‘Gangue’ or ‘Matrix’
in the Ore. An Ore is a combination of two Minerals, the useful Mineral &
the useless Mineral, called the Gangue. Gangue refers to the rocky
impurities like Clay, Sand, Stones & other useless Silicates, which occur along
with the useful Mineral in the Ore deposit, & hence are commercially worthless.
The proportion of Gangue in an Ore varies from one geographical location to
another & likewise the concentration of metals extractable varies
proportionately. The difference between a Mineral & an Ore can be elucidated by
the following example. Bauxite (Al2O3 ∙ 2H2O) &
(Al2O3 ∙ 2SiO2 ∙ 2H2O) are both Minerals of Aluminium. The former is used for
commercial extraction of Aluminium but not Clay. Therefore, Bauxite is an Ore
whereas Clay is not.
COMMON ELEMENTS & THEIR
CORRESPONDING ORE MINERALS
The process of searching for Minerals or Mineral Ore deposits (often called
‘Prospect’) is called Prospecting. Prospecting of Minerals is an integral
initial stage in the overall process of ‘Mineral Exploration’, which is a
more comprehensive & systematic process of Mineral extraction. Mineral
Prospecting is the least disruptive of all Mineral Exploration activities, & it
varies from one Ore deposit to another. The main purpose of Mineral Exploration
is the extraction, beneficiation, & profitable sale of mineral commodities.
Prospecting of Minerals is done by an experienced & certified ‘Geologist’
(also called ‘Prospector’), who use the knowledge of Ore genesis, the knowledge
of known Ore occurrences, & the method of their formation to identify areas of
potential Ore deposits. The chemical property most commonly measured is the
content of a key ‘trace’ element. The main purpose of this step is to
discover zones in the soils or rocks that contain comparatively high
concentrations of particular elements that will guide the ‘Prospector’ to a
hidden Ore deposit. Such conspicuous concentrations of indicator elements in
rocks or soils constitute a ‘Geochemical Anomaly’.
Steps followed in ‘Mineral Exploration are as follows (in the sequential order):
Area selection & Review of Existing Data (Mineral Prospecting)
Ore Beneficiation is an integral preliminary step in the extensive process of
‘Extractive Metallurgy’. Extractive Metallurgy or Metallurgy is the entire
process of obtaining or extracting a pure Metal from its Ore, & is comprised of
3 main operations (as shown below):
General Steps involved in Extraction of Metals from Ores (Extractive Metallurgy)
Minerals are classified, mainly, on the basis of their ‘Chemical Compositions’,
more specifically the dominant Anion or Anionic group (like oxides, sulfates)
present in the Chemical compound characterizing a Mineral. There are mainly
three reasons for selecting such a criterion for classification. One, Minerals
with same Anionic groups share similarities in a lot more properties in
comparison to Minerals with same Cationic groups. The Second reason being the
geological availability of Minerals with same Anionic groups within close
proximity of one another. The Third reason is the existing/contemporary
Nomenclature scheme of Inorganic compounds adopted by various International
Science bodies. A more comprehensive classification, though, can be achieved by
taking into account the internal ‘Crystalline Structure’ of a Mineral. There are
two common Classification Schemes that exist for Minerals, & they are: Dana &
Strunz Classification schemes. The former is more popular & widely followed in
comparison to the latter. Both the Classification Schemes are based on Chemical
Compositions of Minerals. The Dana system of Classification of Minerals is shown
Native Elements: Examples include Gold, Copper, Platinum, Arsenic, Carbon.
Silicates: This is the largest group of Minerals, & the basic Anionic
molecule is SiO4n- Examples include Mica, Quartz, and Amazonite.
Oxides & Hydroxides: These Minerals are formed from the combination of a
Metal with Oxygen or Hydroxide (OH-) group. Examples are Haematite, Bauxite, &
Sulfates: This class of compounds is characterized by the presence of a
Chemical bond between a Metal, Sulfur & Oxygen. These Minerals are formed from
volcanically heated water. These Minerals are soft, pale in colour, & sometimes
transparent or translucent in appearance. Examples are Barite, Gypsum &
Carbonates: This group of Minerals consists of a chemical bond involving a
Metal, Carbon & Oxygen. These Minerals are very soft & dissolve easily even in
mild acids. Examples are Calcite (CaCO3), Malachite, & Rhodochrosite.
Phosphates: These Minerals consist of a Chemical bond between a Metal &
Phosphate (PO43-) group. They are quite rare in occurrence as the other families
of Minerals, & are generally formed as a result of breaking down of other
Minerals by weathering. Such Minerals are soft, brittle, & brightly coloured in
appearance. Examples are Turquoise, Wavellite, & Apatite.
Mineraloids/Non-Minerals: This category encompasses all those compounds which
do not fit properly into any of the above Classes of Minerals, because these do
not fulfill the criterion of having a fixed Chemical composition. Examples are
Opal, Jet, Amber, & Lapis Lazuli.
Apart from the above Classes of Minerals, other Classes containing different
Anionic groups can also be considered, like Borates, Nitrates, Sulfosalts, &
There are only about 8 Minerals out of the 4000-odd total number of all types of
Minerals, which are the major constituents of the 3 types of Rocks: Igneous,
Sedimentary & Metamorphic. They constitute more than 99% of the Earth’s crust, &
are called common ‘Rock-Forming’ Mineral
Class of Mineral
Clear when pure; Impurities cause many Colour variations
Identification & Characterization of Minerals can be done on the basis of their
Physical & Chemical properties. Most of these properties are governed by the type
of elements present in a Mineral, their bonding patterns, & by the Structural arrangement
(Crystalline Structure) of the constituent atoms. Some of these tests (especially
Physical Tests) are simple to conduct, but some can be quite tedious (some Chemical
tests like X-ray Diffraction). A complete diagnosis or identification of a Mineral,
though, can be done by carrying out its Chemical Compositional Analysis. Some of the
‘Common Properties’ evaluated by Mineralogists to identify a Mineral are as follows:
Colour: Colour of a Mineral is defined as the Colour of a fresh,
un-weathered surface observed in White Light. It is one of the simplest properties observable
in a Mineral, but it cannot be used as a ‘Diagnostic’ or Characterizing tool since it is not
a particular Mineral & varies with the level of impurities present. For e.g.,
Malachite always occurs as a green solid but Quartz can occur in a variety of colours, like
purple, yellow, milky etc
Luster: It is a property of a Mineral’s surface which indicates its appearance
(or degree of shining) in reflected light; hence it depends on the illuminating light.
The different types of Luster can be: Metallic, Non-Metallic, Vitreous,
Streak: Streak is defined as the Colour of the powdered Mineral.
This property can serve as a ‘Diagnostic’ tool for Mineral characterization
since it shows the ‘True Colour’ (unaffected by impurities) of the Mineral.
Each Mineral has a Unique Characteristic Streak Colour associated with it.
For e.g., the Mineral Colour of Haematite may be Red or Silver, but its
Streak Colour is always Reddish-Brown. It is determined by rubbing the
Mineral specimen on a tile of Unglazed Porcelain (called Streak Plate).
This test cannot be used for Minerals harder than the Streak Plate
(Mohs Hardness of 7).
Hardness: Hardness of a Mineral refers to its resistance to Scratching or
Penetration. This test is conducted by scratching the unknown Mineral with an
object of known Hardness. The Mohs Scale of Hardness assigns integral values of
Hardness to 10 Minerals, & the Hardness value of any unknown Mineral or substance
is determined on the basis of these values. It is a ‘Relative Scale’ of measurement,
& the Hardness Values only have qualitative significance.
Mohs Hardness Value
Mohs Hardness Value
1.0 – 2.0
2.5 – 3.0
Amalgam, Iron Nail
4.0 – 5.0
Copper Penny, Brass
Knife Blade, Glass
Steel File/Needle, Floor Tile
Streak Plate, Quartz Crystal
Mohs Hardness Scale: The Mohs Hardness of a substance is determined
by observing first what substance can be scratched by it, followed by what
substance can scratch it. The Hardness value is then arrived at by taking an
in-between value between the two values.
Specific Gravity: It is defined as the ratio between the mass of a
particular volume of a Mineral to the mass of an equal volume of water at a
particular temperature (usually 4 0C or 20 0C). This property of Minerals
provides a qualitative measure of the Atomic Weights of the constituent
elements & the nature of internal Packing of the constituent atoms or ions.
For e.g., the nature of Packing is different in Graphite from that of Diamond,
although both have the same Chemical composition (contain Carbon atoms).
The density of packing is a lot more pronounced in Diamond as compared to
that in Graphite. The value of Specific Gravity is also affected by the
presence of impurities in the Mineral sample.
Specific Gravity Values of some common Minerals
Specific Gravity Value
2.60 – 2.75
7.20 – 7.60
Cleavage & Fracture: These two Physical properties (collectively called ‘Breakage’
refer to the reaction of the crystalline structure of a Mineral, in terms of breakage,
to an external force or Stress (strong enough to break it). Simply put, these represent
the manners in which Minerals break (either being dropped from a height or being hard
enough): they can either break along parallel, planar (flat) surfaces – Cleavage, or
they can break irregularly – Fracture. These properties can act as ‘Diagnostic’ tools
for identification of Minerals. The origin of Breakage in Minerals is the presence or
absence of ‘Weak Bonds’ in certain directions in their characteristic atomic arrangements.
‘Weak Bonds’ refer to those planes of atoms along which the bond strengths are lower/weaker
than the surrounding bonds. The presence of Weak Bonds leads to Cleavage, & the absence of
such weaknesses leads to Fracture. Cleavage is quantified by three factors: (1) Quality of
Cleavage, (2) Number of flat surfaces (Cleavages) formed & the angles between them, & (3)
Tenacity: This property is closely related to the Breakage (Cleavage & Fracture)
characteristics of Minerals, since Tenacity gives a measure of the resistance offered
by a Mineral to forces (breaking) of stress or deformation. The difference in the two
properties is that, in the former the nature of surfaces upon breaking is observed,
while in the latter the resistance to breaking is determined. The origin of Tenacity
in Minerals is attributed to the ‘Cohesive’ forces that exist between the constituent
atoms & ions. The Tenacity of a Mineral can be described by the following properties:
(1)Malleability- Capable of being flattened into thin sheets without disintegration
(Example: Gold, Silver, Copper)
(2) Ductile – Capable of being drawn into wires (Example: Gold, Silver, Copper)
(3) Sectile – Capable of being severed by the smooth cut of a sharp object like knife (Example:
Gold, Silver, Copper)
(4) Brittle – Showing little or no resistance to breakage, & hence
disintegrates upon application of force (Most Silicate Minerals are Brittle)
(5) Elastic – Capable of being deformed but original shape or form is restored upon release of the deforming force (Example: Mica)
(6) Flexible – Capable of being bent easily & staying bent after withdrawal of
the force/pressure (Example of Talc)
Magnetism: Those Minerals which contain Iron can be identified by their
attraction to magnets. Magnetite is strongly magnetic, and other Minerals which
exhibit low to moderate strength of magnetism are Ilmenite & Haematite.
Reaction to Dilute Hydrochloric Acid (HCl): This is a Chemical reaction to
positively identify certain Minerals, especially Carbonates, which ‘effervesce’ by
producing Carbon Dioxide gas. For conducting this test 10% HCl is generally used, &
the nature or rate of effervescence varies from one Mineral to another. For e.g.,
Calcite effervesces immediately on addition of HCl, whereas for Dolomite the
effervescence is produced only after the Mineral is crushed into powder.
Besides these properties there are other miscellaneous properties which can
help identify a Mineral. Some of these properties are Crystal Habit, Fluorescence,
Radioactivity, Diaphaneity, & Electrical behaviour.