Minerals

Minerals are defined as naturally occurring, inorganic, solids with a definite chemical composition and a regular, internal crystalline structure. The keys to this definition are the chemical composition and the crystalline structure. Different chemical compositions result in different minerals. A good example is the mineral plagioclase. Plagioclase is a member of the feldspar group, but there is more than one type of plagioclase. Albite and anorthite are two examples. Albite has a chemical composition of  NaAlSi₃O₈, while anorthite's chemical composition is CaAl₂Si₂O₈. Very similar, but different - therefore two different minerals.

Different crystalline structures, or how the atoms and molecules are arranged, result in different minerals. A good example is diamond and graphite. Both minerals are composed of carbon (C). The same chemical composition, but two different crystalline structures - therefore, two different minerals.

Physical Properties

Determination of the actual chemical composition and crystalline structure of a mineral is difficult without the proper equipment. In an introductory level lab it is impossible for us to determine these two aspects of a mineral. Fortunately, these two aspects determine a mineral's physical properties. How the atoms and molecules are arranged and the strength of the bonding between the atoms result in different physical properties for different minerals. While many minerals share common physical properties, when all of a mineral's physical properties are examined, it often results in a unique set of physical properties which can be used to identify the mineral.

Below you will find a chart which defines the physical properties and provides the means for determining the physical property of a mineral sample. These definitions and methods are simplified. Consult your lab manual for detailed discussion.

Mineral Physical Properties Chart
PHYSICAL PROPERTY	Definition*	Testing Method
Cleavage	Breakage of a mineral along planes of weakness in the crystal structure.	Examine the mineral for areas where the mineral is broken. Look for areas where the light reflects from planar surfaces. This can be easily confused with a crystal face and is the most difficult properties for students to master.
Color	Visible light spectrum radiation reflected from a mineral.	Look at the sample and determine its color - white, black, green, clear, etc.
Crystal Form	Geometric shape of a crystal or mineral.	Examine and describe the geometric shape of the mineral - cubic, hexagonal, etc. Not commonly seen in most introductory lab samples.
Fracture	Breakage of a mineral, not along planes of weakness in the crystral structure.	Examine the mineral for areas where the mineral is broken. Describe the breakage as either irregular or conchoidal (has the appearance of broken glass)
Hardness	Resistance to scratching or abrasion.	Use minerals of known hardness from the Mohs Hardness Kits. Scratch the unknown mineral with a known hardness to determine which mineral is harder. Continue doing this with harder or softer minerals from the kit until the hardness is determined.
Luster	Character of the light reflected by a mineral.	Look at the sample to determine if the mineral is metallic in appearance (looks like a chunk of metal) or non-metallic (doesn't look like a chunk of metal).
Magnetism	Electromagnetic force generated by an object or electrical field.	Use a magnet to determine if the magnet is attracted to the sample.
Reaction to HCl	Chemical interaction of hydrochloric acid and calcium carbonate (CaCO3).	Place one small drop of HCl on a sample a watch for a reaction - effervesces (bubbles). Click here to see an short animation (351 Kb)
Specific Gravity	Ratio of the mass of a mineral to the mass of an equal volume of water.	Generally not determined in an introductory lab. Look this information up in your lab manual once the mineral has been identified.
Streak	Color of the mineral when it is powdered.	Grind a small amount of a mineral into a powder on a porcelain streak plate and determine the color of the powder.
Taste	Nerve ending reaction in the tongue to different chemicals.	Lick the mineral. (not recommended in an introductory lab - you don't know who has handled or licked the sample before you).
Other Properties	Fluorescence, Radioactivity	Requires special equipment such as a UV lamp and geiger counter. These are not commonly tested for in an introductory lab.

* Definitions simplified or modified  from Bates, R.L. and J.A. Jackson (eds.), 1987, Glossary of Geology. American Geological Institute, Alexandria, VA, 788 p.

Below is a table listing some of the aspects of the common lab minerals. This table is not a complete listing of all of the physical properties for each mineral. It is designed to highlight those physical properties that are unique to that mineral or assist in identification of that mineral. Be aware that not all mineral samples will necessarily show these physical properties. For example, all plagioclase has cleavage. The sample you examine may or may not show that cleavage. All minerals have a crystal form. However, rarely do introductory mineral samples show a good crystal form. Some types of minerals rarely show a crystal form and even museum collections may not contain good examples of a mineral's crystal form.

Mineral Identification - Diagnostic Physical Properties
Apatite	Green color, H=5, may show hexagonal crystal form
Augite	Dark or dull green color, 2 cleavages at ~90 degrees, similar properties to Hornblende
Biotite	Black color, one perfect direction of cleavage resulting in the mineral pealing into thin, flexible sheets, similar properties to Muscovite
Calcite	H=3, reacts with HCl, 3 directions of cleavage (rhombic cleavage)
Corundum	H=9, often shows hexagonal crystal form
Dolomite	Reacts to HCL in its powdered form, similar properties to calcite
Fluorite	H=4, 4 directions of cleavage, often purple in color (can be white, clear, yellow, green)
Galena	Gray, metallic mineral, 3 directions of cleavage (cubic)
Garnet	Typically reddish brown color, no cleavage, commonly found in twelve-sided crystals (dodecahedrons)
Graphite	"Pencil lead", soft metallic mineral, gray streak
Gypsum	H=2, can be scratched with a fingernail
Halite	"Salt", H=2.5, cannot be scratched with a fingernail, 3 directions of cleavage (cubic), salty taste
Hematite	Reddish brown streak, "rust"
Hornblende	Black to dk. green color, 2 directions of cleavage at 120 or 60 degrees, similar properties to Augite
Magnetite	Magnetic, metallic mineral
Muscovite	Clear or translucent color, one perfect direction of cleavage resulting in the mineral pealing into thin, flexible sheets, similar properties to Biotite
Olivine	Apple green or yellowish green color, H=7 (often difficult to determine), conchoidal fracture, no cleavage
Orthoclase	H=6, salmon pink color is typical, perthitic intergrowths are common, 2 directions of cleavage at 90 degrees, similar properties to plagioclase
Plagioclase	H=6, white or gray color, striations may be seen on cleavage surface, 2 directions of cleavage at 90 degrees, similar properties to orthoclase
Pyrite	"Fool's Gold", gold metallic color
Quartz	H=7, conchoidal fracture, no cleavage, color is typically white or clear but can be pink, red, purple, black
Sulfur	Yellow color, "rotten egg" smell if burned
Talc	H=1, very soft, easily scratched by fingernail

On each of the following pages you will find an image of a mineral and a series of physical properties tests. Identify the physical properties that are present. Once this is done, identify the mineral. It is recommended that you use your lab manual during these exercises as detailed identification information is not given in these web pages. Click each answer, then check to see if you have correctly identified the mineral sample.

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