Minerals are conveniently identified based on their physical properties. These properties include luster, ability to transmit light, color and streak. This lesson will describe each of these properties and examples of each.
We use minerals in our everyday lives.
Most of us are well aware of common uses of minerals, such as copper in electric wiring, gold and silver in jewelry, and even salt or halite in food seasoning. Not all mineral use is well known. For example, graphite is the material used to make pencils, talc is used to make cosmetics and gypsum is used to make wall board. Then there’s silicon, which is derived from quartz and used to make computer chips. The wide variety of minerals begs the question ‘what is a mineral?’ and ‘what properties distinguish one mineral from another?’
Have you ever played the game 20 questions? My family plays this game when we travel. The game begins with the question ‘is it an animal, is it a vegetable, or a mineral?’ In this game, anything not alive is considered to be a mineral.
But, what is a mineral? The term mineral has different meanings for different people. Let’s take a look.My wife is a health nut, and she is concerned with getting her daily vitamins and minerals, for example, calcium, magnesium, and potassium. The mining industry refers to minerals as anything taken from the ground, for example, coal, iron ore, sand and even gravel.Let’s focus on the meaning that geologists apply to minerals, as they make a living studying the earth’s minerals. These scientists will define a mineral as any naturally occurring inorganic solid that has a particular crystalline structure and can be represented by a chemical formula.
These crystalline structures and chemical properties give minerals unique sets of physical and chemical properties shared by all samples of that particular mineral. As the chemical composition of a mineral is difficult to determine, more easily recognized physical properties are commonly used in identification of a mineral. This lesson will identify the different optical properties of minerals used in identification.
Let’s take a look at mineral luster. The appearance, or quality of light reflected from the surface of a mineral, is luster. This is a description of the overall sheen on the surface of the sample. Let me quickly note that luster should not be confused with color. For example, quartz has a glassy luster, but comes in different colors, including purple, rose, and yellow. Therefore, the apparent color of a mineral is generally not used to identify a mineral.
Have you ever wondered why pennies dull over time? A penny contains copper, which has a shiny, or metallic luster. Weathering and corrosion over time can give some minerals, such as copper and galena, a dull appearance, referred to as a submetallic luster.Most minerals don’t look like metals at all, and they exhibit a nonmetallic luster.
Pollucite, for example, has a glassy/vitreous luster, as it looks like glass. Cesium can be extracted from pollucite and used to make capacitors for electronics. Other nonmetallic minerals, such as kaolinite, have an earthy luster. Kaolinite has a dull or clay-like appearance, and is used to produce ceramics, cosmetics and even paper.Stellerite has a pearly luster, as it has the appearance of mother of pearl, which is found on the inside of a clam shell. Stellerite can be used as a chemical filter or sieve, as it contains microscopic channels, allowing water to pass but not larger molecules.
Gypsum has a silky luster like satin cloth and is used to make wall board for construction. Other minerals, such as nepheline, have a greasy luster, or like oil.
The ability to transmit light is another convenient property used in the identification of minerals. Opaque minerals, such as graphite and jasper, cannot transmit light. If both light and an image can be transmitted through a mineral, such as mica, it is said to be transparent.
If light, but not an image can be transmitted through a mineral, such as citrine, it is said to be translucent.
A mineral’s streak is its appearance in a powdered form and can be seen by rubbing it across a piece of porcelain. Unlike the color of the mineral itself, which varies considerably for most minerals, the color of the streak is more consistent of a mineral, and is more helpful in its identification. It is important to note that some minerals, such as quartz, don’t leave streaks, as they are harder than the porcelain. Other minerals, such as cinnabar and pyrite, are similar in apparent color but leave different colored streaks; therefore, the streak is helpful.
In summary, a mineral is any naturally occurring inorganic solid that has a particular crystalline structure and can be represented by a chemical formula. Minerals can be identified based on physical properties. Luster is appearance or quality of light reflected from the surface of the mineral.
Some minerals have metallic or submetallic luster. However, most minerals have a nonmetallic luster, such as glassy, earthy, pearly, silky or even greasy.The ability of a mineral to transmit light is another useful property used in identification of minerals. Opaque minerals cannot transmit light; transparent minerals transmit both light and image, while translucent minerals transmit light but not image.
Finally, a mineral’s streak is its appearance in a powdered form, and it can be used in identification of some minerals that otherwise might appear similar on the surface.
After you see this lesson, you should be able to:
- Discuss how minerals are defined differently by geologists and the mining industry
- Identify the luster of many minerals
- Explain the difference between opaque, transparent and translucent minerals
- Describe what the streak of a mineral is