Technical Information on Minerals & Ores

'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) & Clay (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
ELEMENT IMPORTANT ORE MINERALS
Aluminium (Al) Bauxite, Gibbsite, Boehmite
Beryllium (Be) Beryl
Chromium (Cr) Chromite
Copper (Cu) Malachite, Turquoise, Azurite, Chalcopyrite
Iron (Fe) Haematite, Magnetite, Siderite, Goethite
Lead (Pb) Galena, Cerussite, Anglesite
Manganese (Mn) Pyrolusite, Manganite, Rhodonite, Rhodochrosite
Mercury (Hg) Cinnabar
Molybdenum (Mo) Molybdenite
Nickel (Ni) Pentlandite
Silver (Ag) Acanthite
Tin (Sn) Cassiterite
Titanium (Ti) Ilmenite, Rutile
Uranium (U) Uraninite, Carnotite
Zinc (Zn) Sphalerite

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)
  •   Application for Prospecting Permit
  •   Air borne Survey
  •   Geochemical Survey
  •   Geophysical Survey on Surface
  •   Trenching
  •   Drilling
  •  Environmental Impact Study/ Environmental Impact Assessment (EIA)
  •   Application for Mining Permit
  •   Feasibility Study

There are a number of factors which govern the Profitability of an Ore deposit:


  •   Grade of the Ore (i.e., Percentage of Ore in the Rock)
  •   Extent of the Ore body (i.e., Total amount of Ore present)
  •   Depth of the Ore body
  •   Level of difficulty in removing Underground Water
  •   Presence of any folds or faults
  •   Difficulty in extraction of Metal from the Ore (Smelting). Two aspects areimportant: Amount of Energy Consumption & Method of Extraction
  •   Distance of the Ore deposit from potential markets
  •   Current market price of the Metal extracted
  •   Consideration of Environmental effect (assessment by Environmental Impact Assessment) & impact on Local inhabitants.

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):

Extractive Metallurgy

General Steps involved in Extraction of Metals from Ores (Extractive Metallurgy)

Classification of minerals

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 below:

Native Elements: Examples include Gold, Copper, Platinum, Arsenic, Carbon.  Native Elements
Silicates: This is the largest group of Minerals, & the basic Anionic molecule is SiO4n- Examples include Mica, Quartz, and Amazonite.  Silicates
Oxides & Hydroxides: These Minerals are formed from the combination of a Metal with Oxygen or Hydroxide (OH-) group. Examples are Haematite, Bauxite, & Ruby.  Oxides & Hydroxides
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 & Celesite.  Sulfates
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.  Carbonates
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.  Phosphates
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.  Mineraloids Non Minerals

Apart from the above Classes of Minerals, other Classes containing different Anionic groups can also be considered, like Borates, Nitrates, Sulfosalts, & Tungstates.

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


Rock-Forming Mineral Class of Mineral Significant Diagnostic Properties
Mohs Hardness Colour Luster Cleavage
Quartz Silicate 7.0 Clear when pure; Impurities cause many Colour variations Vitreous None
Feldspar (Orthoclase, Plagioclase) Silicate 6.0 Orthoclase: Pink, Cream Plagioclase: White, Grey Vitreous 2 at ~900
Biotite Silicate 2.5 Black Vitreous: sometimes appears Metallic 1
Muscovite Silicate 2.5 White or Clear
1
Amphibole Silicate 5.5 Black Vitreouseous 2 at 1200
Olivine Silicate 6.5 Green Vitreous Vitreous
Pyroxene Silicate 5.5 Black Vitreous 2 at 870 & 930
Calcite Carbonate 3.0 White or Clear Vitreous 3 Not at 900
Clay Silicate 2.5 White Dull None

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 constant
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
  Colour of Mineral
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, and Earthy.  Luster of Mineral
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).  Streak of Mineral
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.
Standard Mineral Mohs Hardness Value   Common Objects Mohs Hardness Value
(Approx.)
Talc {Mg3Si4O10(OH)2} 1 Pencil Lead 1.0 – 2.0
Gypsum (CaSO4·2H2O) 2 Fingernail 2.5
Calcite (CaCO3) 3 Gold, Silver 2.5 – 3.0
Fluorite (CaF2) 4 Chalk 3.0
Apatite {Ca5(PO4)3(OH,Cl,F)} 5 Amalgam, Iron Nail 4.0 – 5.0
Orthoclase (KAlSi3O8) 6 Copper Penny, Brass 3.5
Quartz (SiO2) 7 Tooth Enamel 5.0
Topaz {Al2SiO4(OH,F)2} 8 Knife Blade, Glass 5.5
Corundum (Al2O3) 9 Pumice 6.0
Diamond (C) 10 Steel File/Needle, Floor Tile 6.5
  Streak Plate, Quartz Crystal 7.0
Garnet Paper 7.5
Tungsten Carbide 9.0
Silicon Carbide

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
Mineral Specific Gravity Value
Borax 1.70
Halite (NaCl) 2.10
Graphite 2.23
Gypsum 2.30
Quartz 2.65
Feldspar 2.60 – 2.75
Calcite 2.72
Muscovite Mica 2.80
Diamond 3.51
Corundum 4.0
Haematite 5.26
Galena 7.20 – 7.60
Cinnabar 8.0
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) Cleavage Habit.  Cleavage & Fracture

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.