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There are several physical properties used to indentify minerals and depending on the mineral one property can be more diagnostic than another. Click on a feature for more information.


CLEAVAGE
  

COLOUR


FORM AND HABIT


HARDNESS


LUSTRE


STREAK











CLEAVAGE


Many minerals break along well defined planes depending on its internal structure and the crystal system to which the mineral belongs. This property is known as cleavage and there are several 
 varieties. For example, Galena which belongs to the cubic crystal system shows excellent cleavage
 in three directions parallel to the faces of a cube. Fluorite which is also part of the cubic system illustrates octahedral cleavage parallel to the faces of a octahedron. Other common cleavages
 include those parallel to the faces of a prism and pyramid.







COLOUR


This is obviously the most charactersitic feature of minerals but it is not always the most useful.
 Many minerals have different colours (allochromatic) due to the impurities they contain, for example quartz. Some minerals though always have the same colour (idiochromatic), for example malachite (green), azurite (blue) and sulphur (yellow).







FORM AND HABIT


Most minerals will form good crystals if they are allowed to grow uninterrupted. Many minerals regularly form well developed crystals however some don't, therefore allowing the study of crystal morphology to lead to the identifiication different minerals. FORM describes the set of faces
 produced by the symmetry of the crystal. Crystals of the same mineral may show more than one
 form, for example pyrite which has both cubic and octahedral crystals. HABIT is related to the appearance of single crystals or crystal aggregates, there are several descriptive terms.

ACICULAR: describes fine, neddle shaped crystals.
BLADED: describes crystals that are flat
 like a knife blade.
BOTRYOIDAL: crystals which are shaped like a bunch of grapes.
FIBROUS: describes crystals which are shaped like fine threads.
MASSIVE: for crystals with no regular form.
RENIFORM: crystals shaped like a kidney.
TABULAR: describes crystals with broad,
 flat surfaces.







HARDNESS


This property is the measure of a mineral's ability to resist scratcing or abrasion. It is represented
 by a number between 1 and 10, according to a reference set of minerals. Known as the Moh's
 scale after the mineralogist Friederich Moh, 1 represents the softest while 10 is the hardest.
 Each mineral in the list can scratch any other with a lower number but can only be scratched by
 those with a higher number. The reference set is as follows:




1 : Talc,  2 : Gypsum,  3 : Calcite,  4 : Fluorite,  5 : Apatite,

6 : Orthoclase,  7 : Quartz,  8 : Topaz,   9 : Corundum,  10 : Diamond




The hardness, therefore, of an unknown mineral is determined by which minerals it will scratch
 which will scratch it. Obviously this reference set is not always going to be available so other tests
 be carried out using a fingernail: with a hardness of 2-2.5, copper coin: with a hardness of 3 or a
knife blade: with a hardness of 5.5-6.5.










LUSTRE


This property refers to the amount and quality of light being reflected from the surface of a mineral. 
tallic lustre is produced by opaque minerals that absorb a lot of light, for example the majority
the metals. A non-metallic lustre, of which there are several varities, is produced by transparent minerals. For example: 
ADAMANTINE: is a lustre with a high degree of sparkle e.g. Diamond.
EARTHY: minerals that do not show a lustre are said to be dull or earthly.
PEARLY: is a lustre
similar to that of a pearl and is caused by the relection of light from parallel surfaces.

RESINOUS: is a lustre similar to that of resin.
SILKY: is a lustre similar to that of silk

VITREOUS: is the lustre of broken glass e.g. Quartz.








STREAK


Streak is the mineral in powdered form and can be produced by scratching the mineral against a
 piece of porcelain or scratching the surface of the mineral with a knife. The colour of the streak is consistent and does not vary with the colour of the mineral. It can be said that idiochromatic
 minerals have a streak which is lighter than the mineral colour while allochromatic minerals have a
 white or greyish white streak. There are exceptions though for example, pyrite which produces a yellow/black streak and hematite which can be grey/black in colour yet produce a red streak. 






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