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After reading this article you will learn about the physical properties of minerals.
Physical Properties of Minerals Depending on Light:
(A) Form:
Minerals assume different shapes, which depend upon the internal structure of the minerals. Different terms have been used to describe the forms of different minerals.
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The important terms that are commonly used are:
1. Fibrous:
When the mineral has a thread like structure.
Example:
Asbestos.
2. Columnar:
When the mineral has a thick or thin columnar structure.
Example:
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Hornblende.
3. Granular:
When the mineral has numerous grains, coarse or fine.
Example:
Calcite, Chromite.
4. Crystalline:
When the mineral has fine crystals packed together.
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Example:
Galena, Pyrite.
5. Massive:
When the mineral has an irregular structure.
Example:
Feldspar.
6. Bladed:
When a mineral appears to be composed of a blade like structure.
Example:
Kyanite.
7. Acicular:
When the mineral consists of thin, sharp and slender needles as shown in the figure.
Example:
Natrolite.
8. Botryoidal:
Rounded aggregates of minerals like a bunch of grapes.
Example:
Chalcedony.
9. Foliated:
When the mineral consists of thin separable sheets.
Example:
Mica.
10. Radiating:
When the fibres or needles are arranged around a central point.
Example:
Iron Pyrite.
11. Tabular:
The mineral is flat rather than elongated as shown in the figure.
Example:
Calcite, Orthoclase.
12. Reniform:
When the mineral possesses rounded prominences like those of a Kidney as shown in the figure.
Example:
Hematite.
(B) Colour:
Colour is an important physical property of minerals, which depends upon light. The colour of any mineral depends upon the absorption of some and reflection of others of the colour of white light. If the mineral absorbs all the colour of white light, it appears black.
If the mineral reflects all the colours of white light it appears white.
A mineral appears red when it can absorb all the colours of white light except red colour.
Similarly, a mineral appears green when it can absorb all the colours of white light except green colour.
(C) Streak:
Streak is nothing but the colour of the mineral in its powdery form. Rubbing the mineral against the streak plates can get streak. Some minerals will have the same colour from their massive form also in their powdery form.
For example, Natural gold is Yellow in both in its colour and powder form. There are some other minerals, which have different colour form their massive form to their powdery form.
For example, the mineral pyrite which is commonly called as “Fool’s Gold” is yellow in colour in its massive form but it gives a Black streak. Similarly, haematite, the ore of Iron will give red or brown grayish brown in colour but it gives Cherry red Streak.
(D) Diaphaneity:
Diaphaneity means ability to transmit light.
The terms used are:
(i) Transparent:
When the mineral allows the light to pass through it. In the case of transparent minerals the objects can be clearly seen through such minerals.
Example:
Quartz and Calcite (Coloured varieties).
(ii) Translucent:
When the minerals allow only a part of the light to pass through. The outlines of any object cannot be seen clearly through such lines of translucent mineral.
Example:
Quartz and Calcite (Milky white varieties).
(iii) Opaque:
When a mineral does not allow any light to pass through. The objects are not seen through opaque mineral.
Example:
Bauxite, Haematite and Magnetite.
(E) Lustre:
The lustre of a mineral is its appearance in a reflected light, which is independent of its colour.
The terms used are:
(i) Adamantine:
When a mineral has lustre like Diamond.
Example:
Zircon, Diamond, Sulphur, etc.
(ii) Resinous:
When a mineral has lustre like Grease.
Example:
Opal amber and a variety of Zincblende.
(iii) Vitreous:
When a mineral has lustre like Glass.
Example:
Quartz, Calcite, and in many other Silicate Minerals.
(iv) Pearly:
When a mineral has lustre like Pearls.
Example:
Talc, Brucite, Micas, etc.
(v) Metallic:
When a mineral has lustre like metals.
Example:
Galena, Pyrite, and Chalcopyrite.
(vi) Silky Lustre:
Mineral with a Silky shine.
Example:
Asbestos.
(vii) Dull:
When a mineral has no lustre.
Example:
Bauxite.
Physical Properties of Minerals Depending on State of Aggregation:
(A) Cleavage:
Cleavage of the mineral is its tendency to split along certain parallel planes producing more or less smooth surface.
Cleavage lines are the weak lines or divisional planes in a mineral.
The terms used are:
(i) Perfect, Good or Distinct:
When a mineral can split up with great ease and give a smooth surface.
Example:
Mica, Feldspar.
(ii) Imperfect, Poor, Imperfect or None:
When a mineral does not split up with an average force.
Example:
Quartz.
(B) Fracture:
Fracture of the mineral may be defined as the appearance of its broken surface, when the mineral is hammered and broken.
The terms used are:
(i) Even fracture,
(ii) Uneven fracture,
(iii) Conchoidal fracture,
(iv) Hackly fracture, and
(v) Earthy fracture.
(i) Even fracture:
Appearance of a mineral in its broken surface is smooth.
Mineral examples:
Chert, Mica.
(ii) Uneven fracture:
When the mineral breaks with very rough and coarse surfaces.
Mineral examples:
Chromite and various other minerals.
(iii) Conchoidal fracture:
When a mineral breaks with curved surfaces or concentric rings or half-moon shape.
Mineral example:
Quartz.
(iv) Hackly fracture:
When a mineral breaks with irregular surfaces having sharp edges.
Mineral example:
Native copper.
(v) Earthy fracture:
When the broken surface is soft and almost smooth.
Mineral example:
Chalk.
(C) Hardness:
The hardness of a mineral is the resistance. It offers to abrasion, which is determined by observing the comparative ease or difficulty in scratching it with another mineral of known hardness.
Moh’s Scale of Hardness given below always expresses it:
A mineral with lowest hardness is talc and the mineral with the maximum hardness is diamond. It has been observed that a soft mineral like Talc and Gypsum can be scratched with a finger nail. A steel knife can cut Apatite and Orthoclase but not Quartz. The average hardness of a normal finger nail may be up to 2.5 whereas the hardness of pen knife is 6.5.
Physical Properties of Minerals Depending on Specific Gravity:
The specific gravity of a mineral is the ratio of its weight to the weight of equal volume of water.
Specific gravity is depending on the weight of the specimen, palm is the judge.
Low—Light minerals (less weight).
Medium—Intermediate minerals (medium weight).
High—Heavy minerals (much weight).
Physical Properties of Minerals Depending on Certain Senses such as Taste, Feel and Odour:
Taste:
The terms used are:
(i) Alkaline:
Taste of soda
(ii) Bitter:
Taste of epsom salt
(iii) Cooling:
Taste of saltpetre
(iv) Saline:
Taste of common salt
(v) Sour:
Taste of sulphuric acid
2. Feel:
Feel is the sensation upon touching or handling minerals.
The terms used are:
(i) Greasy.
(ii) Soapy.
(iii) Rough.
(iv) Harsh.
3. Odour:
Some characteristic smell when rubbed breathed upon or heated.
The terms used are:
(i) Arsenical.
(ii) Sulphurous.
(iii) Argillaceous.
(i) Arsenical:
Like the Odour of Garlic.
Example:
Orpiment.
(ii) Sulphurous:
Like the Odour of burning Sulphur.
Example:
Pyrite.
(iii) Argillaceous:
Like the Odour of clay.
Special Properties of Minerals:
Special properties of minerals such as:
(i) Magnetism.
(ii) Reaction with dil. HCl acid.
Magnetism:
Only few minerals are attracted by a Bar magnet or Horseshoe magnet. Such minerals are called Ferromagnetic. The most common minerals that are attracted by a magnet are Magnetite, Pyrrhotite, etc.
Based on the strength of the magnetism, the minerals can be grouped under the following headings:
(i) Highly Magnetic:
Example:
Magnetite, Pyrrhotite.
(ii) Moderate Magnetic:
Example:
Siderite, Iron Garnet, ilmenite and Haematite.
(iii) Weakly Magnetic:
Example:
Tourmaline, Spinel, and Monazite.
(iv) Non Magnetic:
Example:
Calcite, Quartz, and Feldspars.
Reaction with Acid:
Acid test:
Scratch a fresh even surface of the given mineral until grooved and powdered. Certain carbonate minerals react with Hydrochloric acid.
Example:
Calcite CaCO3 is got. Add one or two drops of dil. HCl carefully to it, immediate effervescence (fizzing) confirms Carbonate especially CaCO3
