Study of Physical properties of minerals
Objective:- Study of Physical properties of minerals
Theory :- Earth is made up of minerals that are the constituents of rocks. Mineral specimens are usually identified by determining their physical properties.
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Colour : Although the colour of some minerals, such as azurite, is quite distinctive, other minerals, such as quartz, occur in a variety of colours. Also there are many white minerals. Hence colour is frequently NOT a useful diagnostic property.
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Streak: Streak is the colour of the powdered mineral. It is a useful diagnostic property for many coloured minerals — especially those with a metallic lustre. It is found by rubbing the specimen on a piece of unglazed tile, or streak plate.
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Lustre: The lustre of a mineral is the way its surface shines when held up to the light. Lustre is a property distinct from colour. There are many ways of classifying and describing lustre, but the following system is adequate:
Vitreous — the mineral shines like glass — e.g. quartz, diamond
Metallic — the mineral shines like the surface of a metal — e.g. pyrite, galena
Earthy (dull) – the mineral does not shine at all — e.g. kaolinite
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Hardness: The hardness of any mineral can be assigned a number between 1 and 10, on Moh's Scale of Hardness. The instruments used to determine the hardness of a mineral specimen are (in order of increasing hardness) a finger-nail, copper coin, knife blade and a quartz crystal.
The table below lists the minerals that define Moh's Scale of Hardness, and gives the relative hardnesses of the test items named above.
Density: It is not usual to measure the actual densities (relative to water = 1) of specimens; however, minerals should be classified according to whether they are light, medium or heavy. This can be done by holding similar-sized specimens of two different minerals in your hands, and comparing their weights.
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Cleavage: When a piece of a mineral is dropped or struck, it may tend to break so that flat, shiny surfaces are formed.
Minerals, or individual surfaces, that do not cleave to form flat faces are said to show fracture.
Cleavage is a diagnostic property for identification of minerals, but the cleavage of an actual specimen is not always easy to determine. Many specimens do not show the expected cleavage characteristics.
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Magnetism: Some minerals that contain iron are magnetic. Magnetite is strongly magnetic, and will be attracted by a magnet. Other iron-bearing minerals such as ilmenite sand size particles.
Reaction to dilute Hydrochloric Acid: Some minerals especially carbonates, effervesce when a drop of dilute hydrochloric acid is placed on them. This is useful diagnostic test for calcite and a white mineral which is not easily distinguished.