Introduction
Eight primary mineral properties are commonly used to identify a particular mineral. These include: color, density, hardness, streak, luster, cleavage, fracture and crystal form. However, some of these properties are more reliable than others for identifying a specific mineral.
The least reliable property for identifying a mineral is color. This is because many minerals can occur in a variety of colors. For example, the mineral quartz can be white, yellow, pink or purple; while the mineral calcite can be white, gray, yellow or brown. Therefore, color alone is not always a good indicator of what mineral you have.
Other less reliable properties for identifying minerals include streak (the color of a mineral’s powder), luster (the way light reflects off the surface of a mineral) and cleavage (the way a mineral breaks along certain planes). While these properties can be helpful in some cases, they are not always reliable indicators of what type of mineral you have.
The most reliable properties for identifying minerals are density, hardness and crystal form. These properties are less likely to be affected by environmental factors and will usually remain the same regardless of where the mineral is found. By using all three of these properties together, you can usually identify a particular mineral with a high degree of accuracy.
The Three Commonly Used Mineral Properties
Mohs’ Hardness, streak, and specific gravity are all properties that are commonly used to identify minerals. However, each of these properties can be unreliable at times. For example, two minerals may have the same Mohs’ hardness, making it difficult to tell them apart.
Hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by German geologist and mineralogist Friedrich Mohs and is one of the qualitative scales with which mineral hardness is commonly expressed.
Color
Color is perhaps the most visually apparent of the rock-forming minerals. Although color alone cannot be used to identify a mineral, it can sometimes be useful in combination with other properties such as density, hardness, and cleavage. Some minerals are multi-colored, and may exhibit bands or zones of different colors. Many minerals owe their color to impurities or defects in their crystal structure; for example, smithsonite is colored by zinc impurities, and turquoise by copper. Some minerals are colored by the presence of trace elements that substitute for atoms of a different size in the crystal structure; for example, chromium substitutes for aluminum in some emeralds, giving them their green color.
The most common colors of rock-forming minerals are:
-Black (or very dark gray)
This color is due to the presence of graphite or other carbon-bearing materials. Minerals that are black or very dark gray include biotite mica,Obsidian and tourmaline.
-White (or colorless)
Many minerals are white because they do not contain any coloring agents. Others may be white because they are made up of decayed organic matter (such as bone or pearl), or because they contain tiny bubbles of gas. Minerals that are white or colorless include quartz, diamond, gypsum and talc.
-Red and pink
These colors are due to the presence of iron oxides such as hematite or limonite. Minerals that are red or pink include Garnet, spinel and tourmaline.
-Green
This color is usually due to the presence of copper in the crystal structure. Minerals that are green include emerald, malachite and turquoise.
-Blue and violet
These colors are also due to trace elements in the crystal structure; for example, lazurite contains sulfur and aluminum, giving it its characteristic blue color. Other blue and violet minerals include fluorite, lapis lazuli and sodalite.
Streak
Streak is the color of a mineral in powder form when it is rubbed on a white plate. The color may differ from the actual mineral sample due to impurities. Mineralogists use streak to help identify minerals, since many minerals occur in more than one color.
The Least Reliable Mineral Property
Color is the least reliable mineral property because other factors such as impurities or translucency can affect what color you see. Also, some minerals can come in a variety of colors. For example, quartz can be clear, white, pink, purple, or gray.
Cleavage
Cleavage is the tendency of a mineral to split along certain flat planes of atoms giving the mineral its characteristic shape. For instance, halite (rock salt) always splits into octahedrons (eight-sided figures). Some minerals have perfect cleavage, meaning the flat planes are at 90 degrees from each other. Other minerals have poor or imperfect cleavage, which may be at angles close to but not exactly 90 degrees.
The reliability of cleavage as an identification tool depends on two factors: the degree of perfection and the number of flat planes. For example, consider graphite. Graphite has perfect cleavage in one direction and poor cleavage in a perpendicular direction. So, if you were to look at graphite under a microscope, you would see that it breaks into thin sheets. This is why pencil lead is made of graphite—it can be easily drawn into a fine point. Since graphite has only one direction of perfect cleavage, it is not a very reliable identification tool. On the other hand, halite has perfect cleavage in three directions forming octahedrons. This means that no matter how you look at halite under a microscope, it will always break into these octahedral shapes. Therefore, cleavage is a much more reliable identification tool for halite than for graphite.
In general, the more directions of perfect cleavage and the greater the degree of perfection, the more reliable Cleavage is as an identification tool.
Conclusion
When using physical properties to identify minerals, hardness is the least reliable property. This is because there is no definitive scale for hardness, and because different minerals can have the same hardness but be otherwise entirely different. For example, both quartz and topaz have a hardness of 7 on the Mohs scale, but they have completely different chemical compositions, colors and streak colors.