Mineral Identification: Properties and Structure
Classified in Chemistry
Written on in 
English with a size of 6.43 KB
Mineral Definition and Fundamental Properties
Minerals are defined as: naturally occurring, inorganic, solids with a definite chemical composition, a regular crystalline structure, and specific and consistent physical properties. The fundamental aspects of this definition are a mineral's chemical composition, crystalline structure, and physical properties.
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. For instance, albite and anorthite are two examples. Albite has a chemical composition of NaAlSi3O8, while anorthite's chemical composition is CaAl2Si2O8. Though very similar, their distinct compositions define them as two different minerals.
Similarly, different crystalline structures—or how the atoms and molecules are arranged—also result in different minerals. A classic example is diamond and graphite. Both minerals are composed solely of carbon (C). Despite having the same chemical composition, their vastly different crystalline structures make them two distinct minerals.
Identifying Minerals by Physical Properties
Determining the actual chemical composition and crystalline structure of a mineral is challenging without specialized equipment. In an introductory lab setting, it is typically impossible to ascertain these two aspects directly, as the process requires advanced techniques such as X-ray diffraction, mass spectrometry, and SEM analysis.
Fortunately, a mineral's composition and structure directly determine its physical properties. The arrangement of atoms and molecules, along with the strength of the bonding between them, results in distinct physical properties for different minerals. While many minerals may share common physical properties, when all of a mineral's physical properties are examined collectively, a unique set can often be identified, which can then be used to identify the mineral.
Note: This identification process is an iterative, trial-and-error method that has been in use for thousands of years and is still extensively employed today in field research and various industrial settings.
Below is a chart defining common physical properties and outlining simplified methods for their determination in a mineral sample. For more detailed discussion, consult your instructor.
Mineral Physical Properties Chart | ||
Physical Property | Definition | Testing Method |
Cleavage | Breakage of a mineral along planes of weakness in its crystal structure. | Examine the mineral for broken areas. Look for planar surfaces that reflect light. This can be easily confused with a crystal face and is often the most difficult property for students to master. |
Color | Visible light spectrum radiation reflected from a mineral. | Observe the sample and determine its color. |
Fracture | Breakage of a mineral not along planes of weakness in its crystal structure. | Examine the mineral for broken areas. Describe the breakage as either irregular or conchoidal (like broken glass). |
Hardness | Resistance to scratching. | Use minerals of known hardness or items from a Mohs hardness kit. |
Luster | The character of light reflected by the mineral's surface. | Observe the sample to determine if its appearance is metallic, non-metallic, pearly, vitreous (glass-like), greasy, dull, etc. |
Magnetism | Electromagnetic force generated by an object or electrical field. | Place a magnet on the mineral to observe if there is a reaction. |
Reaction to Hydrochloric Acid | Chemical interaction of HCl and calcium carbonate (CaCO3). | Drop HCl on the sample and observe if it effervesces (bubbles). |
Specific Gravity | Ratio of the mass of a mineral to the mass of an equal volume of water. | Place the mineral in water and measure the amount of water displaced. |
Smell | Some minerals, such as sulfur, have a unique smell. | Smell the mineral to determine if there is a diagnostic odor. |
Streak | Color of the mineral when it is powdered. | Grind the mineral against a streak plate to see if it leaves a mark. (Try both a white and black streak plate) |
Taste | Nerve ending reaction in the tongue to different chemicals. | Lick the mineral to determine if there is a diagnostic taste. |
Understanding Mineral Cleavage
Cleavage describes how a mineral breaks along various planes. If you strike a mineral with three planes of cleavage, it will likely break along these planes, resulting in smaller pieces that also display three planes of cleavage. This property is directly related to the arrangement of atoms within the mineral’s crystal lattice. Both the number of cleavage planes and the angle at which these planes intersect represent important diagnostic properties. Cleavage planes may be visible in both hand samples and thin sections.