Identifying Minerals

Crystal FormSome minerals can form distinctive crystalline shapes that make them very recognizable. Ex: Halite (cubic), Quartz (Six-sided and pointed).

Not all minerals will form perfect crystals and so identification based only on crystal form is rare.

LusterThe way that a mineral reflects light from its surface is called luster.Two types: Metallic and Non-metallic luster.Metallic: silver, gold, copperNon-Metallic: They do not shine like metals. Instead, they could be described as dull, pearly, waxy.Rather subjective and so luster should be used in conjunction with another process.

HardnessA very useful and reliable test for identifying minerals.Hardness is a measure of how easily a mineral can be scratched.The German geologist Friedrich Mohs developed a scale by which an unknown mineral’s hardness can be compared to the known hardness of ten minerals.

Cleavage & FractureThe arrangement of atoms within a crystal will determine how a mineral will break. Minerals break along planes where atomic bonding is weak.A mineral that splits relatively easily and evenly along one or more flat planes is said to have cleavage. (ex: Mica and Halite)

Minerals that break with rough or jagged edges are said to have fracture. Ex: Flint (conchoidal fracture)

Cleavage & Fracture

StreakStreak is the colour of the mineral when it is broken up and powdered. This can usually be seen by running a mineral across an unglazed porcelain plate.Non-metallic minerals – usually whiteMetallic minerals – varies (most useful for metallic minerals)Can you use streak to identify all minerals? Why or why not?

ColourA very noticeable feature of a mineral and yet one of the least useful.Colour is often the result of trace elements within the mineral.Ex: Quartz (Amethyst, Rose Quartz, Red Jasper, Citrine)

Density and Specific Gravity

Density = mass/volumeDensity does not depend on the size and shape of a mineral. It only depends on the internal atomic structure. For this reason, it is a useful identification tool.If the sample is too small, it is difficult to determine the density by conventional means. Therefore, geologists typically measure density by determining a materials specific gravity.

Specific gravity – ratio of the mass of a substance to the mass of an equal volume of water at 4 degrees Celsius.

TextureHow does a mineral feel to the touch?

Similar to luster, this is a subjective measurement. Ex: Talc is greasy and Fluorite is smooth.

Special PropertiesDouble Refraction – Iceland Spar (Calcite)Effervescence – Calcite will fizz when exposed to an acid (release of carbon dioxide)Magnetism – Magnetite (iron containing minerals)Iridescence – Labradorite (caused by the bending of light rays)Fluorescence – Fluorite, Calcite (glows in the dark when exposed to UV light)