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Mineralogy

1.Definition of a Mineral:  

A mineral is naturally formed, homogeneous inorganic solid substance, which possess distinctive physical properties, definite chemical composition and definite internal atomic structure.

Physical properties of Minerals: The study of physical properties of minerals is important, since it varies from mineral to mineral. Following are important physical properties.

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PROPERTIES DEPEND UPON LIGHT:

  1. Colour

  2. Streak

  3. Luster

  4. Transparency (Diaphaneity)

 

PROPERTIES DEPEND UPON THE INTERNAL ATOMIC STRUCTURE AND MODE OF FORMATION:

  1. Habit/Form/Structure

  2. Hardness

  3. Fracture

  4. Cleavage

  5. Specific gravity

 

PROPERTIES DEPEND UPON SENSES:

  1. Taste

  2. Odor, feel

  3. Acid reaction, Double refraction,

  4. Magnetism

 

TOOLS FOR TESTING AND IDENTIFICATION OF MINERALS

  1. Hand lens

  2. Penknife

  3. Magnet

  4. Streak plate

  5. Dilute HCL (Hydrochloric acid)

 

1.PROPERTIES DEPEND UPON LIGHT:

  1. COLOUR: The colour shown by a mineral depends upon the absorption of some and the reflection of others of the colored rays. The colour of mineral is one of the most important physical property, some minerals show distinctive colour and are as follows:

 

  • Blue : Azurite, Saderiteite,

  • Green : Flurite, Malachite, Talc, Microcline,

  • Yellow : Sulphur, Chalcopyrite, pyrite.

  • Red : Jaspar, orthoclase, Cinnabar, Garnet.

  • Lead grey : Galena, Graphite.

  • Colorless : Quartz and Zeolite.

 

However, the colour of non-metallic minerals vary greatly and the variations of colour in minerals may be due to, Surface alterations,ii)Difference in chemical composition,Presence of impurities,Inclusion of some other mineral matters etc.,

 

B) STREAK: Streak is the colour of powdered mineral and may be obtained by rubbing the fresh mineral surface on unglazed porcelain plate called “Streak plate”. The streak may be of different colour from the mineral colour. Table showing the different colours in streak of minerals.

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C) LUSTURE (LUSTRE): The appearance of the shinning surface of a mineral in reflected light is called luster. The intensity of luster is influenced by transparency, reflectivity and surface structure of the mineral. It is also depends upon the absorption and reflection of ordinary light. The luster of minerals can be divided into two types:

1) Metallic luster. 2) Non-metallic luster.

1) Metallic luster: A mineral having the appearance of a metal reflection. Ex: Galena, Pyrite,

2) Non metallic luster: All minerals dull appearance have a non-metallic luster.

i) Vitreous: Luster shown by broken glasses: Quartz, Feldspars.

ii) Resinous or Waxy: Luster shown by a resin or wax. Ex: Sphalerite, Chlorite.

iii) Greasy luster: Luster shown by a thin layer of oil. Ex: Agate, Mica, Gypsum etc.

iv) Pearly luster: Mineral surface like pearl shine. Ex: Talc, Muscovite, Calcite.

v) Silky luster: Luster shown by silk or fiber surface minerals. Ex: Asbestos, Gypsum.

vi) Adamantine luster: Brilliant surface reflection. Ex: Diamond.

vii) Dull or Earthy luster: When a mineral shows no luster. Ex: Bauxite, Kaolin.

 

D) TRANSPERANCY: The property possessed by some minerals to transmit light is known as Transparency/ diaphaneity. The terms used to express the varying degrees of this property are

Transparent: When the outlines of an object seen clearly though the mineral, Ex: Rock crystal,

Translucent: A mineral which can transmit light only at edges. Ex: Calcite, Fluorite, Quartz,

Opaque: When no light is transmitted through the mineral. Ex: Hematite, Magnetite, etc.,

2.PROPERTIES DEPEND UPON INTERNAL ATOMIC STRUCTURE AND MODE OF FORMATION:

Habit/Form/Structure: The characteristic natural internal shape of minerals is called as habit/form. Mineral when formed under favorable physic- chemical conditions occurs as well developed individual crystals, crystal aggregates, with various stages of growth or distorted crystals.

Minerals assume various types of forms which are not necessarily dependent on the crystal characters. These forms are described by various terms as given below:

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Hardness: Hardness may be defined as the resistance offered by the mineral to scratching, rubbing or breaking. The degree of hardness is determined by observing the comparative easy or difficulty with which one mineral is scratched by the other or by a penknife. Hardness varies from mineral to mineral.

A German mineralogist Friedrich Moh’s, was conducted several experiment found that some minerals according to their increasing hardness. This series commonly known as “Moh’s scale of hardness” and is as follows. “Moh’s scale of hardness”

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Fracture: The nature of the broken surface in minerals other than the direction is called fracture. But minerals with very good cleavages break more readily along cleavage planes thus making it difficult to observe the fracture, Common types of fracture.

Conchoidal: The broken mineral surface shows curved concentric shells. Ex: Quartz, Magnesite,

Sub Conchoidal: A lesser degree of curved surface of concentric shells. Ex: Jasper

Hackly: when the mineral breaks with an irregular surface and edges. Ex: native copper,

Uneven: When mineral breaks with a rough irregular surface. Ex: Magnesite, feldspars.

Even: When mineral breaks with a smooth and regular flat surface. Ex: Flint.

Earthy: No fracture surface observed on the minerals surface. Ex: Kaolin, Bauxite.

Cleavage: It is the property of certain minerals to break along certain planes called the cleavage planes. These cleavage planes are always parallel to crystal faces of the mineral. Minerals may split/break in one, two or three directions. The cleavage planes, which run parallel to one another belongs to cleavages. Thus the mineral exhibits one Set, (micas) two sets, (Feldspars) and three sets. (Calcite and Galena)

Basal cleavage: One set of cleavages, which can easily split into sheets. Ex: Muscovite mica.

Prismatic cleav: Two sets of cleavages planes parallel crystal faces. Ex: feldspars

Cubic cleavage: Two sets of cleavages at right angles to each other. Ex: Galena, pyrite

Rhombohedral cleav: Three sets of cleavages at right angles to each other.Ex: Calcite.

Octahedral cleavage: Four sets of cleavages. Planes are parallel to faces. Ex: Fluorite.

 

Specific Gravity: Minerals vary considerably in weight, some being heavy for their size, others light. The relative weight of a mineral is known as its specific gravity which is a number that expresses the ratio between its weight and the weight of an equal volume of water. Specific gravity can be calculated in the following equation:

Specific Gravity = Wa / (Wa - Ww)

Where, Wa = Weight of mineral specimen in air

Ww = Weight of mineral specimen in water

 

The specific gravity of a mineral depends upon their chemical composition and molecular weight. The most practical method uses are Jolly’s spring yard balance. Beam balance, Walkers steel yard etc., depends upon the size of the minerals. But for practical purposes, specific gravity can be roughly estimated by using the relative terms as

Low (2.5 and less): Less weight. Ex: Graphite.

Medium (3.5 and 3.5): Medium weight. Ex: Quartz, feldspar, Calcite.

High (3.5 above): Heavy weight. Ex: Silver.

 

3.PHYSICAL PROPERTIES OF MINERALS DEPENDS UPON CERTAIN SENSES AND SPECIAL PROPERTIES:

 

Taste: When the minerals are soluble in water. Ex: a) saline taste b) Alkaline taste of soda.

Odor: Minerals have characteristic odors rubbed upon or heated. Ex: Sulphurous .

Feel: Smooth, Greasy, Rough are some of the kinds of feel of minerals that may help in their identification. Ex: Talc - Smooth surface, Mica - Greasy surface, Agate - Rough surface, Native copper - Harsh surface

Acid reaction: Certain carbonate minerals reacts with hydrochloric acid. Ex: Calcite.

Magnetism : Some minerals attracts magnet. Ex: Magnetit

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