Mrs. Lucas- Physical Properties of Minerals

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.

• flammability

• density

• streak

• magnetism

Quartz is a mineral, but oil is not. Minerals must be naturally occurring, not man-made. This applies to both quartz and oil. But minerals also must be solid. Quartz is a solid, but oil is a liquid. Minerals cannot be alive or come from anything that was part of something living. Quartz meets this requirement. Oil, however, is made of tiny plants and animals that became buried over millions of years. As pure substances, minerals can be described by chemical formulas. A mineral’s composition won’t vary. Some minerals are just one element, like gold and zinc, and can be found on the periodic table of elements. Other minerals are compounds that contain combinations of the same elements in the same amounts. Quartz is a compound. Every piece of quartz has the same composition with the chemical formula SiO2. Oil is a mixture, which isn’t a pure substance and cannot be described by a chemical formula. Lastly, minerals are crystalline. Their atoms are arranged in a repeated structure. You can often see the crystals in quartz. Oil doesn’t have a crystalline structure.

Which of the following would be a clue that a substance is a mineral?

• it appears on the periodic table of elements

• it exists as a liquid or gas

• it comes from a specific type of flower

• it was made by a machine in a factory

Rocks are solids that are made of minerals. For example, schist is a rock that may contain the minerals muscovite, chlorite and talc, among others. Obsidian may contain hematite, biotite and other minerals, too. While minerals are pure substances, rocks are not. Not every piece of schist will contain the same minerals bonded in the same way. Think of rocks like cookies with chocolate chips, oatmeal, raisins and walnuts. Minerals are like the ingredients in those cookies: the flour, the sugar, the chocolate, oats, raisins and nuts. Not every cookie will have the same ingredients even if you’re using the same batter. That’s why rocks aren’t pure substances. They’re mixtures. You can’t see every ingredient in a cookie. Likewise, you can’t always see every mineral that makes up a rock. That’s why we can observe and measure minerals to identify them, but it’s harder to do the same for rocks.

Rocks are

• are always pure substances

• made up of different substances

• edible like cookies

• always made the same minerals bonded the same way

Color is a physical property of minerals. Different minerals have different colors. For example, gold—of course—is gold! However, color isn’t always a reliable property to identify a mineral. Fool’s gold, or pyrite, has a yellowish-gold color too. Some minerals come in different colors. Garnet, for example, can be red, green, yellow, orange, brown, pink, purple, gray or black. Some minerals come in all colors. Some minerals can also tarnish or oxidize, which means they can become discolored in certain environments. Silver and copper can both become discolored in this way. Silver tarnishes black, yellow or brown. Copper oxidizes green.

The color of a mineral

• is always a result of oxidation

• is hard to use as an identifying property

• is an example of a chemical property

• is never the same color of another material

Streak may be a more reliable property than color to help you identify a mineral. Streak is the color of a mineral when it’s powdered. Even a mineral that comes in different colors will have the same color streak. That means that a mineral’s streak might be different from its main color. Calcite, for example, may be white, yellow, brown, orange, pink or many other colors. It can also be multicolored. But calcite always has a white streak. Streak can help distinguish minerals that have the same or similar main color. You can test a mineral’s streak by rubbing it against a streak plate or a white tile.

When might a streak test be useful?

• when a mineral comes in different colors

• when a mineral is multicolored

• when two minerals have similar main colors

• all of the above

Luster is the way light is reflected by a mineral’s surface and how brilliant or dull it is. A mineral’s luster can be metallic or nonmetallic. Metallic means that a mineral is shiny and looks like metal. Galena and gold have metallic luster. There are different types of nonmetallic luster. Some nonmetallic minerals have a vitreous, or glassy, luster. Others have a pearly luster. Still others have a greasy luster that looks oily. Diamonds have what is called an adamantine luster. That’s a rare luster that makes diamonds sparkle.

Minerals that are oily-looking have a(n)

• vitreous luster

• metallic luster

• greasy luster

• adamantine luster

Hardness is another physical property of minerals. The harder a mineral is, the less likely it is to be scratched. To determine a mineral’s hardness, scratch it against an object with a known hardness. You could try a penny or even your fingernail. We can measure hardness using the Mohs scale. The Mohs scale goes from 1 to 10. Minerals with a lower number are softer. Minerals with a higher number are harder. Talc has a hardness of 1, so it can be scratched very easily. Diamond has a hardness of 10. No other mineral can scratch it. A penny has a hardness of 3.5, and glass has a hardness between 5.5 and 6.

Which of the following could be the Mohs scale number for a mineral that is harder than talc, but softer than a penny?

• 7

• 0.4

• 4.5

• 2

Cleavage and fracture are properties that describe how minerals break. Cleavage describes how a mineral breaks into even, flat surfaces. A mineral’s cleavage is measured by quality and number of sides. A mineral has perfect cleavage if it leaves no rough surface at all when broken. Cleavage is poor if there is more of a rough surface created. Some minerals only have cleavage along one side. Others have cleavage on two or more sides. Feldspar has two cleavage planes that intersect at right angles.

Fracture describes how a mineral breaks into surfaces that aren’t flat or even. Some minerals can have a splintery fracture, like pectolite. Others have an earthy fracture. They break into crumb-like pieces, like soil. Other minerals have fracture that is rounded like a shell. Sulfur has this fracture, which is called conchoidal.

The cleavage of a mineral

• must happen on two or more sides of a mineral

• is perfect if it creates a smooth, flat surface

• cannot create 90 degree , right angles

• can occur in rounded conchoidal shapes