Rock Physical Property Tests

Rocks are made of one or more minerals. Minerals are pure, solid, inorganic (nonliving) materials found in Earth's crust. Minerals are made of one or more elements. Elements are the most basic, naturally occurring substances on Earth. Elements cannot be broken down (except by radioactive decay).

If you think of a cookie as a rock, the flour, sugar, and chocolate chips are like the minerals that make up the rock. Depending on the recipe, you get different kinds of cookies. It is the same with rocks because each type of rock has a different combination (or recipe) of minerals.

Minerals all have chemical compositions and physical properties unique to that specific mineral. (A chocolate chip in an oatmeal cookie is the same as the chocolate chip in a peanut butter cookie). Even rocks with the same mineral ingredients may be different due to variations in the amounts of minerals (more flour, fewer chocolate chips) and the processes by which they are formed such as being burned, doughy, or just right. Common rock-forming minerals are feldspar, quartz, calcite, mica, and hornblende.

All minerals have value, but their value varies. The more rare a mineral is, the more valuable it is. The same goes with mineral use. If a mineral is used for many of different things like copper, it becomes valuable. Some minerals are mined for their beautiful properties, such as diamonds and other gems. Some are so valuable they are used for jewelry or decorations, like gold and platinum.

We use different characteristics to identify people, such as eye color, hair color, language, etc. Geologists do the same thing, using specific properties to identify rocks and minerals. Geologists use the following tests to distinguish minerals and the rocks they make: hardness, color, streak, luster, cleavage and chemical reaction.

A scratch test developed by a German mineralogist Fredriech Mohs in 1822 is used to determine mineral hardness. He developed a hardness scale that helps to identify mineral properties. The scale measures hardness on a scale of 1-10. One being the softest mineral (talc) and 10 being the hardest mineral (diamond). Common objects of known hardness can be used to determine mineral hardness. These common objects are: your fingernail (2.5), a penny (3), a piece of glass (6) and a knife blade or nail. For example, if your fingernail can scratch the mineral, it has a hardness of less than 2.5, which is quite soft. If the mineral can scratch glass it has a hardness of greater than 6, which is very hard.

Color can sometimes be helpful when identifying minerals. However, some minerals have more than one color, like quartz. Quartz can be blue, brown, pink, red, purple, and almost any other color, or it can be totally colorless. Therefore, geologists have developed a better way of using color as an identifying property. This property is called a streak.

Streak is the name given to the colored residue left by scratching a mineral across an abrasive surface, such as a tile of unglazed porcelain. The streak may not always be the same color you see in the hand specimen. A mineral with more than one color will always leave a certain color of streak. Hematite is a mineral that can be red, brown, or black, but it will always leave a characteristic reddish brown streak.

Another mineral property that geologists use to identify minerals is luster. Luster is the way in which the surface of a mineral reflects light. There are two main types of luster: metallic and nonmetallic. A metallic luster is shiny and similar to the reflection from a metal object, such as a faucet. A mineral that does not shine like metal has a nonmetallic luster. For example, the wall has a nonmetallic luster. There are many types of nonmetallic luster. A glassy luster is bright and reflects light like a piece of glass. A greasy luster has an oily appearance. An earthy luster is a very dull and looks like dirt. Waxy luster looks like the shininess of a crayon.

Cleavage is another property used to distinguish minerals. Cleavage is the tendency for minerals to break along flat planar surfaces. Cleavage is rated as good, fair and poor depending on the quality of the flat surface produced. Mica, for example, is a mineral that has good cleavage. It breaks into very flat sheets. Minerals that have very poor cleavage will only break along irregular surfaces. Quartz, for example, will break into pieces that have a seashell-like fracture plane. Others, like garnet, shatter with no distinguishable pattern. These are considered to have no cleavage at all.

A weak acid is used to tell if rocks or minerals contain calcium carbonate (CaCO3). If the specimen fizzes (giving off CO2) when it comes in contact with acid, it is considered carbonate rich. If it does not contain calcium carbonate, it will not fizz. Calcite and aragonite are two minerals that will always fizz.