To identify minerals, scientists look at their properties. Properties are attributes or traits that are shared by all members of a group. Some properties are chemical and some are physical. Unlike chemical properties, physical properties can be measured and observed without changing a substance’s composition, or what it’s made of. There are a lot of physical properties. Some examples include density, conductivity and magnetism. Minerals can be described and identified by their physical properties. Some of the properties we use to describe minerals include color, luster, hardness, streak, cleavage and fracture.
Which of the following is not an example of a physical property?
A
flammability
B
density
C
streak
D
magnetism
Answers
Answer:
B
density
D
magnetism
C
streak
Explanation:
Reading: Physical Characteristics of Minerals
What are Minerals?
All rocks except obsidian and coal are made of minerals. (Obsidian is a volcanic rock made of glass and coal is made of organic carbon.) Most rocks contain several minerals in a mixture characteristic of the particular rock type. When identifying a rock you must first identify the individual minerals that make up that rock.
Minerals are naturally occurring, inorganic solids with a definite chemical composition and a crystal lattice structure. Although thousands of minerals in the earth have been identified, just ten minerals make up most of the volume of the earth’s crust—plagioclase, quartz, orthoclase, amphibole, pyroxene, olivine, calcite, biotite, garnet, and clay.
Together, the chemical formula (the types and proportions of the chemical elements) and the crystal lattice (the geometry of how the atoms are arranged and bonded together) determine the physical properties of minerals.
The chemical formula and crystal lattice of a mineral can only be determined in a laboratory, but by examining a mineral and determining several of its physical properties, you can identify the mineral. First, you need to become familiar with the physical properties of minerals and how to recognize them.
Minerals can be identified by their physical characteristics. The physical properties of minerals are related to their chemical composition and bonding. Some characteristics, such as a mineral’s hardness, are more useful for mineral identification. Color is readily observable and certainly obvious, but it is usually less reliable than other physical properties.
How are Minerals Identified?
Figure 1. This mineral has shiny, gold, cubic crystals with striations, so it is pyrite.
Figure 1. This mineral has shiny, gold, cubic crystals with striations, so it is pyrite.
Mineralogists are scientists who study minerals. One of the things mineralogists must do is identify and categorize minerals. While a mineralogist might use a high-powered microscope to identify some minerals, most are recognizable using physical properties.
Check out the mineral in figure 1. What is the mineral’s color? What is its shape? Are the individual crystals shiny or dull? Are there lines (striations) running across the minerals?
Color, Streak, and Luster
Diamonds are popular gemstones because the way they reflect light makes them very sparkly. Turquoise is prized for its striking greenish-blue color. Notice that specific terms are being used to describe the appearance of minerals.
Color
Figure 2. This mineral is shiny, very soft, heavy, and gold in color, and is actually gold.
Figure 2. This mineral is shiny, very soft, heavy, and gold in color, and is actually gold.
Color is often useful, but should not be relied upon. Different minerals may be the same color. Real gold, as seen in figure 2, is very similar in color to the pyrite in figure 1.
Luster
Luster describes the reflection of light off a mineral’s surface. Mineralogists have special terms to describe luster. One simple way to classify luster is based on whether the mineral is metallic or non-metallic. Minerals that are opaque and shiny, such as pyrite, have a metallic luster. Minerals such as quartz have a non-metallic luster.
Luster is how the surface of a mineral reflects light. It is not the same thing as color, so it crucial to distinguish luster from color. For example, a mineral described as “shiny yellow” is being described in terms of luster (“shiny”) and color (“yellow”), which are two different physical properties. Standard names for luster include metallic, glassy, pearly, silky, greasy, and dull. It is often useful to first determine if a mineral has a metallic luster. A metallic luster means shiny like polished metal. For example cleaned polished pieces of chrome, steel, titanium, copper, and brass all exhibit metallic luster as do many other minerals. Of the nonmetallic lusters, glassy is the most common and means the surface of the mineral reflects light like glass. Pearly luster is important in identifying the feldspars, which are the most common type of mineral. Pearly luster refers to a subtle irridescence or color play in the reflected light, same way pearls reflect light. Silky means relecting light with a silk-like sheen. Greasy luster looks similar to the luster of solidified bacon grease. Minerals with dull luster reflect very little light. Identifying luster takes a little practice. Remember to distinguish luster from color.