Mineral Properties
There are thousands of different minerals, and each of those minerals has a unique set of physical properties, These are not only useful for identifying minerals, but also for determining potential industrial uses of the minerals. A few are listed below.
Crystal Form
Mineral shape, if the mineral is allowed to grow unrestricted.
Controlled by crystalline structure.
Well developed crystals are relatively rare, so this may not help with mineral identification.
Making it more complicated, many minerals have more than one crystal form.
Color
Color is the most obvious physical property and most deceptive.
It is common for minerals to have more than one color.
Color is controlled by chemical composition - sometimes trace chemicals can impart color.
Streak
All minerals have a streak color, it’s controlled by chemical composition.
The color of a mineral that is powdered.
Streak color of a mineral shows less variation than its color.
Most minerals have only one streak color.
Luster
Appearance and intensity of light reflected from the mineral surface.
Two main groups:
Metallic - looks like metal.
Non-metallic - doesn't look like metal.
Non-metallic descriptions include silky, pearly, earthy, adamantine, vitreous, resinous.
Controlled by chemical composition and crystal structure.
Generally, not very useful for identification.
Fracture
Definition: manner in which a mineral breaks, but not along a plane of weakness.
All minerals have fracture. It’s controlled by crystal structures and chemical bonding.
Similar to cleavage
Conchoidal fracture - break looks like broken glass.
Irregular fracture (self-descriptive)
Cleavage
Tendency of a mineral to break along a plane of weakness within crystalline structure.
Controlled by crystalline structure and chemical bonding.
0, 1, 2, 3, 4, and 6 cleavage planes can exist.
Most difficult physical property for students to understand and identify.
Hardness
Ability of a mineral to resist abrasion.
Controlled by the crystalline structure and the type of chemical bonds.
stronger bonds = harder mineral
Measured using Mohs Scale of Hardness
Other physical properties, that can definitively identify some unique minerals:
Magnetism
Some minerals contain enough iron (other metals) to generate an electromagnetic force.
Running a small magnet across a mineral, one can feel if there is an attraction.
Taste
Strange as it sounds, some minerals can be identified by taste (e.g., halite, or salt).
Do not lick introductory lab specimens (could be poisonous, and who knows who, before you?)
Hydrochloric acid
Dilute HCl reacts with Calcite (CaCO3) bubbling off CO2, distinguishing from Dolomite
Acids are dangerous, and can damage some valuable minerals. Be careful.
Radioactivity
Not a common physical property
Useful in some sedimentary rocks to identify presence of radioactive minerals.
Collectors use Geiger counter for detecting unusual minerals in pegmatites.
Specific Gravity
Ratio of the mass of a given volume of mineral to the mass of an equal volume of water
Not measured directly in introductory, but the “heft” of certain minerals is distinctive
Galena (Pb) is a good example
Fluorescence
Some minerals luminesce (“glow”) in presence of ultraviolet light from excited electrons
Short-wave ultraviolet lamps (100-280nm) can cause blindness and should not be viewed directly