The Mineral Celestite
Luster and Hardness
Seven properties are commonly used to identify minerals: color, luster, hardness, streak, cleavage, fracture, and crystal form. For many of the minerals you will look at, these properties will be all that is necessary to identify the samples. However, at times other properties such as reaction to acid, magnetism, striation, and the existence of exsolution lamellae will be needed to make a correct identification.
The luster of a mineral is a description of how a mineral reflects light. This can be described by either a metallic luster or a nonmetallic luster. If a mineral has a metallic luster, it will appear to have a silvery, gold, brassy, or iron look it. Nonmetallic minerals will appear more earthy or glassy. When observing luster, it is important to observe a freshly broken surface of the mineral since oxidation can occur on exposed surfaces and the mineral will become tarnished.
The hardness of a mineral is a measure of the mineral's resistance to abrasion and therefore a measurement of the strength of the chemical bonds between the individual atoms or molecules within the mineral. The strength of the bonds depends solely on the physical properties of the atoms within the bond. With over 2000 known minerals, each of which has its own definite chemical composition, using hardness as an identifying property could be a daunting task.
Mohs Scale of Hardness
This process is simplified by using a relative scale such as Mohs Scale of Hardness.
To use this technique, hardness values between 1 and 10 have been assigned to some common minerals. 1, the softest, is a mineral known as talc; the hardest mineral, 10 on Mohs scale, is diamond. Along with assigning hardness values to common minerals, these values have also been assigned to other common objects as illustrated below.
Minerals or objects that have higher hardness values will be able to scratch minerals or objects of lesser hardness values. Therefore fluorite, hardness 4, will be scratched by any mineral with hardness greater than 4 as well as by an iron nail.
To determine the hardness of a mineral, start by scratching it with your fingernail then use progressively harder and harder objects until the mineral scratches. You will then be able to narrow down the hardness of the mineral to a hardness range.