Geology Collection / Mineral Testing & Identification

 

The following are the tools and properties used to identify minerals. A mineral is a naturally occurring, usually inorganic, crystallized chemical element or compound, with a definite chemical composition which varies only within specific limits. Although compounds can be produced in the lab that have many of the characteristics of a mineral, because they are not naturally occurring they are not minerals. Nearly all minerals are inorganic, that is, not produced by living organisms. Minerals are crystallized, a solid with a definite internally ordered arrangement of atoms. Therefore, liquids and gases are not minerals. Because a mineral has a definite chemical composition, its composition can be expressed as a specific chemical formula. An example is quartz, which is composed of silicon and oxygen and has the formula SiO2.

 

Tools

 

Click thumbnail for larger image.
 

Porcelain Streak Plate
Hardness: 6

Magnifying Lenses
Glass Plate
Hardness: 5.5
Magnets
Knife
Hardness: 5.5
HCl

Nail
Hardness: 5.5

   

 

Properties

 

Hardness - the resistance of a mineral to scratching. It is measured on the Mohs hardness scale from 1 (softest) to 10 (hardest).
Cleavage - the breaking of a mineral along its crystallographic planes (planes of atomic weakness). Because minerals are crystalline, these planes are regular and repeat at very closely spaced intervals. Minerals with cleavage always cleave in the same manner no matter how many times they are broken or how small the cleavage fragments are. Hold the mineral up to the light - light reflects off a smooth cleavage face. Some specimens may seem to have flat cleavage surfaces, but you just can't decide if it is cleavage. In these cases, look for another flat surface running the same way within or on the other side of the specimen. One cleavage surface is virtually always accompanied by others.
Fracture - this occurs when the chemical bonds are of equal strength in all directions. When the mineral is broken, an irregular breakage results. A common type is conchoidal fracture, like the curved break of broken glass. Fracture may also be very rough and irregular. Hold the mineral up to the light - if it is just fractured, it will appear dull.
Streak - the color of a mineral in its powdered form, usually obtained by rubbing the mineral on a streak plate and observing the mark it leaves. The porcelain streak plate has a hardness of about 7 and cannot be used on minerals of greater hardness. Some minerals will crumble when rubbed against a streak plate. Make sure the mineral is truly powdered.
Luster - the reflection of light from the surface of a mineral, described by its quality and intensity. There are two major types of luster; metallic which looks like a metal and may be bright or dull and nonmetallic which does not look like a metal and can be described as shiny, glassy (vitreous), earthy, pearly, greasy, or waxy. Bright metallic minerals tend to have a black or very dark streak. The streak of nonmetallic minerals is colorless or very light in color and probably not of much use in mineral identification.
Striations - some minerals have closely spaced fine lines on cleavage faces that look like straight grooves. These lines result from the intergrowth of two crystals called twinning.
Magnetism - the magnet may suspend the mineral or just pull at it. Certain minerals show a stronger reaction than others. Magnetic minerals have a metallic luster.
Acid Test - using a dilute hydrochloric acid solution, minerals containing carbonates will effervesce (fizz and form bubbles) when the acid solution is placed on them. Some minerals need to be in the powdered form to produce the same reaction. Rinse specimen under water after observing the reaction. If the mineral is a carbonate, the acid will continue to dissolve it, if not rinsed.
Color - minerals come in many colors, and for some minerals, color may be used to help identify them. But color is very unreliable. Some minerals characteristically exhibit a variety of colors. Never rely on color as the only basis for mineral identification.
Specific Gravity - the ratio of the weight of a given volume of a substance to the weight of an equal volume of water. The specific gravity of water is 1. Because it is a ratio, it is a unitless quantity. Some minerals are obviously much heavier than others and noting the heavier specimens, may be useful.

 

Begin Identification Process

 

Please have all properties tested and recorded before beginning the identification process. Print out a copy of the Sample Sheet for use.

Click to begin identification of minerals. Have all testing and recording done first.