Optical and Physical Properties

Light waves reflect as they bounce off, or refract as they bend through, all matter.  We visually perceive mineral optical properties, including but not limited to: color, metallic luster, or sheen.  One mineral may exhibit many colors due to impurities included in the molecular matrix, or to grain size refraction, making mineral classification by color alone confusing at best.  Optical identification using refractive liquids or a microscope is possible in a laboratory, but not practical for the field.  We use physical properties to identify minerals in the field.  Useful physical properties for identifying common minerals include crystal form and habit, cleavage and fracture patterns, luster, color and streak, hardness, density and specific gravity, magnetism, and chemical properties such as reaction to mild acids.

Tools for Field Identification

Field Identification

Crystal form is the rare result of unhindered growth in an open space, for example, a pyrite cube.  More often, the only way to identify several differently shaped crystals of the same mineral by form involves comparing the angles of the crystal faces, such as quartz.  Crystal habit is common growth trait, including threads for asbestos and plates for micas.

Cleavage is a preferred mineral breakage pattern along smooth planes with weak atomic bonds, such as sheets for micas and rhombs for calcite.  A lack of cleavage is a fracture, which exhibits a curved or conchoidal pattern like broken glass, as seen in garnet and quartz.

Luster, which is the amount and visual quality of the light reflected off a mineral, includes the descriptive terms metallic, resinous, vitreous or glassy, adamantine or diamond-like, greasy, and pearly. 

Color should not be a primary means of identifying minerals, as stated above, while streak on an unglazed white porcelain tile is a reliable color effect.  Hematite can be reddish brown or metallic silver in visual form, but always streaks bright red. 

Hardness is the resistance to scratching.  The Mohs scale of hardness uses a ten mineral scale, which ranges from1 for softest talc to 10 for hardest diamond.  Common objects can be used in the field to test for hardness.  Gypsum, a Mohs 2, is scratched with a fingernail; calcite, a Mohs 3, with a copper penny; potassium feldspar, a Mohs 6, with a pocketknife or piece of glass.  If a mineral scratches a common object, the mineral is harder than the corresponding common object's Mohs scale number. 

Density, how heavy or light an object feels, is measured in units of mass per volume.  Silicates weigh 2.5 to 3 grams per cubic centimeter (g/cm3); galena, 7.5 g/cm3; gold, 19 g/cm3.  Silicates feel lighter than metals of the same volume.  Specific gravity is a unit-less number comparison of water, density of 1 g/cm3, to an equal volume of a mineral.  Silicate minerals have a specific gravity range from 2.5 to 3; galena, 7.5; and gold, 19. 

Two metals, magnetite and pyrrhotite, are highly magnetic and attracted to a small iron magnet. 

For chemical properties, halite tastes salty.  Calcite bubbles strongly and dolomite weakly when sprayed with diluted hydrochloric acid, due to carbon dioxide released in a chemical reaction by the acid.  I do not recommend the taste test due to the danger of poisoning by some toxic minerals, but generations of geology students have at least tried tasting minerals without apparent damage.  Likewise acids can cause tissue burns and must be handled with great care.

Mineral Formation and Associations

Minerals often form as a result of specific conditions or associations with specific geologic activities.  For example, halite and calcite often form as the result of evaporative and chemical sedimentary processes in shallow seas.  Quartz, silicates and metal crystals grow as veins or in host rock cavities from a moving mineral-rich hydrothermal or hot water solution that forms due to groundwater in contact with an igneous magma body.  Garnets and micas form from clay minerals compressed and heated by metamorphic processes associated with deep burial or tectonic activities associated with movement of crustal plates. 

I recommend obtaining pocket guides that can be carried in the field, or left in a vehicle for later use.  First on the list is a field guide containing the physical properties for rocks and minerals.  Note that the guide minerals pictured are usually perfect specimens that are rarely found hence the need to identify physical properties for proper mineral identification.  Pocket field guides for collecting sites and geologic research papers give clues to locations for specific minerals.  A pocket guide of geologic terms will help decipher the language of the research papers.  A pocket ten-power microscope or jewelers loupe will reveal details not evident to the unaided eye.  With practice, you can identify many minerals without guides, but everyone eventually finds an unusual mineral that requires a reference book.  And even more rarely, someone finds a new mineral previously not described in the literature. 

As Spiderman realized that with great power comes great responsibility, so also does our knowledge of the power of the atom, the super hero of the mineral world, require even greater responsibility for the future of mankind. 

Thanks to Dr. Bob Shuster, my geology professor at the University of Nebraska at Omaha; Physical Geology, Skinner and Porter, John Wiley & Sons; Mineralogy for Amateurs, Sinkankas, Van Nortstran Reinhold; and Spiderman, Columbia Tristar Home Entertainment.