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Posted on Jul 13, 2023 in Language

Minerals

Minerals are defined as: naturally occurring, inorganic, solids with definite chemical composition, regular crystalline structure, and specific and consistent physical properties. The keys to this definition are the chemical composition and the crystalline structure and the 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 of plagioclase. Albite and anorthite are two examples. Albite has a chemical composition of NaAlSi₃0₈, while anorthite’s chemical composition is CaA1₂Si₂0₈. 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.

Part A: 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 (this process requires use of x-ray diffraction, mass spectroscopy, SEM analysis and so on and so forth).

Fortunately, the two aspects listed above (composition and structure) 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, we can very often come up with a unique set of physical properties which can be used to identify the mineral. (Note: this process is very much a repeat, trial and error, simple method that has been in use for thousands of years, and is still used extensively today in field research and in many industrial settings.)

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 TA 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.
Fracture	Breakage of a mineral, not along planes of weakness in the crystal structure.	Examine the mineral for areas where the mineral is broken. Describe the breakage as either irregular or conchoidal. (like broken glass)
Hardness	Resistance to scratching.	Use known minerals of known hardness, or by using items from a Mohs’ hardness kit.
Luster	Character of the light reflected by the mineral.	Look at the sample to determine if it is metallic, non-metallic, pearly, vitreous (like glass), greasy, dull, etc. in appearance.
Magnetism	Electromagnetic force generated by an object or electrical field.	Use a magnet and place it on the mineral to see if there is a reaction.
Reaction to Hydrochloric Acid	Chemical interaction of HCl and calcium carbonate. (CaCO3)	Drop HCl on the sample and see 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 smell.
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.

Part B: Cleveage

Cleavage: Description of how a mineral breaks along various planes. If you smash a rock hammer against a mineral with 3 planes of cleavage, it will likely break along its cleavage planes, resulting in smaller pieces which will also display 3 planes of cleavage. This is directly related to the structure of atoms in the mineral’s lattice. Both the number of cleavage planes and the angle at which these planes intersect represent important properties. Cleavage planes may be visible in both hand sample and thin section