The hardness of minerals is often a characteristic property, and as such is useful in identification. Hardness is defined as the resistance of a mineral to scratching, thus soft minerals such as talc powder easily.
In 1824 an Austrian mineralogist F. Mohs selected 10 minerals that displayed increasing hardness. These are shown here; this scale which is known as Mohs’ Scale is universally used.
It is important to realise that the steps between the minerals are unequal: diamond is 100’s of times harder than talc. Useful and convenient standards for estimating mineral hardness are your fingernail (2.5), a penknife blade (5), and window glass (6.5).
Some minerals show a tendency to break along particular planes, which are parallel to possible crystal faces. These develop along planes of weakness in the atomic lattice.
The degree of perfection of the resultant surfaces is a measure of the relative strength of the bonds holding these planes together.
Mica has an eminent cleavage (breakage is difficult to prevent) parallel to one plane, while calcite (seen above in Mohs’ Scale) cleaves easily into rhombs.
A mineral is said to have a fracture when it breaks in a regular pattern. The nature of the surface is often characteristic of the mineral type. It is independent of cleavage and not related to the atomic structure at all.
Freshly fractured surfaces are useful as they will show the true mineral colour, as against the surface colour which may have become altered.
Even – the surface is reasonably flat.
Conchoidal – mineral breaks with a curved surface.
Uneven – where the surface is rough.
Hackly – the surface carries small sharp irregularities.
Specific gravity is a measure of the weight of a material compared to the weight of an equal volume of water, while density is mass per unit volume and measured in grams per cubic centimetre. The two are often used synonymously which is technically incorrect. Today specific gravity is more often quoted than density.
Dark minerals usually have a greater specific gravity than light coloured minerals, and the same is true of metallic minerals as compared to silicate minerals.
|Specific gravity of some common minerals|
|Halite 2.16||Muscovite 2.8-3.0||Haematite 4.2|
|Gypsum 2.32||Hornblende 3.2||Pyrite 5.01|
|Feldspar 2.56-2.7||Topaz 3.6||Magnetite 5.2|
|Quartz 2.65||Garnet 3.7-4.3||Galena 7.5|
|Calcite 2.71||Barytes 4.5|
Colour in minerals is caused by the selective absorption of parts of the white light spectrum. This may be due to the influence of impurities, or to the atomic structure of the mineral.
Although colour may be variable in certain minerals it is an important characteristic for the identification of others. The green of malachite, the blue of azurite, and the yellow of sulfur are so diagnostic of those minerals that no other characters are needed for their identification.
This is the colour of the powdered mineral, when rubbed on a piece of unglazed porcelain. Only minerals with a hardness of less than 7 will streak easily. In many cases the colour of a mineral will be quite variable, whereas the colour of its streak is reasonably consistent. This will often differ from the mineral colour. The streak is a useful characteristic for distinguishing between some oxides of iron. It is less useful for silicates which usually have a white streak or are else too hard.
This is the nature of the light reflected back from the surface of the mineral, and varies according to the quantity and quality of the light. It is easily discerned by examination and is the product of two properties. These are the atomic nature of the crystal lattice, and secondly, the refractive and absorptive indices of the mineral.
Lustres are divided into metallic lustre and non-metallic. The latter are most varied, and fall in intensity from splendent (as in diamond), through vitreous (quartz), to greasy (nepheline) and finally to dull or no lustre (chlorite).