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Properties of Minerals


How do we identify minerals?

          Minerals have eight physical properties that are used to identify them: Color, Luster, Hardness, Streak, Density, Crystal Shape, Cleavage and Fracture, and Special Properties.


·        Color: The color of a mineral is easily observed, however many minerals vary in color (quartz) and many minerals have similar colors.  Malachite has a unique green color and Azurite has a unique blue color, so these minerals are easily identified by color.  Color is not a reliable way to identify minerals because many factors can affect color: sun, heat, cold, pollution, radiation, and water.


·        Luster: This is how a mineral reflects light. Luster can be described as; metallic, brilliant, glassy, pearly, silky, and dull


·        Hardness: This is how a mineral resists being scratched.  German scientist (Friedrich Moh) determined a scale of minerals from the softest Talc with a 1 to the hardest mineral Diamond with a 10.  You can determine the hardness of any mineral by comparing it to the ten minerals on Moh’s Harness scale.  Any higher numbered mineral will scratch a mineral lower.  The Field hardness scale is not as reliable because none of the tests are compared to a standard.


·        Streak: This refers to the color of the powder scraped off a mineral.  Streak can be an excellent identifier.  Even if the color of a mineral varies its streak does not.  We use a streak plate for this test.  A streak plate has a hardness of 7.


·        Density: This is the amount of matter in a given space.  The density of a mineral is always the same.  You can compare the densities of similar sized minerals, the denser mineral feels heavier.  Density is calculated by the formula


·        Crystal Shape: There are six basic shapes of crystals.  Every mineral has a definite crystal shape.


·        Cleavage and Fracture:  These terms refer to the way a mineral breaks.  A mineral is said to have cleavage if it splits smoothly along definite surfaces.  Halite cleaves in three directions causing it to break into cubes.  Micas cleave in one direction causing it to break into sheets.  Most minerals do not cleave, they fracture, break along jagged or rough surfaces.


·        Special Properties:  Some minerals have unique properties.  For example Magnetite is magnetic, Fluorite glows under ultraviolet radiation, Halite tastes salty, Sulfur smells like rotten eggs, uraninite is radioactive.  These are just some examples.