Physical properties of Minerals

AIM
To study the 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.

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.

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.

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.

Hardness: The hardness of any mineral can be assigned a number between 1 and 10, on Moh's Scale of Hardness. 
Moh's Scale of Hardness

Hardness No.

Minerals

 

1

Talc

 

2

Gypsum

 

3

Calcite

Finger nail ~2.5

4

Fluorite

Copper coin ~3.5

5

Apatite

Knife blade ~5.5

6

Orthoclase(Felspar)

7

Quartz

8

Topaz

9

Corundum

10

Diamond(Kimberlite)

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

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.

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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