ROCKS

How do geoscientists define rocks?

Let’s begin with a simple definition of a rock. A rock is a consolidated mixture of minerals. At the beginning of this module, a rock called quartzite was introduced. Quartzite is composed of a mixture of minerals that are predominantly quartz. In other words, many pieces of the same mineral (quartz) are held together (consolidated) to form the rock quartzite. However, rocks can also consist of different combinations of minerals. An example of a rock with many different types of minerals is the rock granite, the most common rock in continental landmasses. The granite shown in the Figure below consists of four different types of minerals that are all silicates. Silicates are the Earth’s largest and most complex family of minerals. The elements listed in the four chemical formulas shown below include all of the eight most abundant elements in Earth’s crust.  

Photograph of the rock granite
Figure 1. Photograph of the rock granite and minerals that comprise it. Mineral names and chemical formulas are shown.

We now know that a rock is a consolidated mixture of minerals, but what holds it together? Referring back to the rock granite, mineral crystals interlock together to create a texture called crystalline. This crystalline texture is best seen under the microscope, but is also apparent in many hand samples.

Crystalline texture of a granite shown in a hand sample and under a microscope
Figure 2. Crystalline texture of a granite shown in a hand sample and under a microscope. Also shown is a sketch of what is viewed under the microscope.

Do all rocks have a crystalline texture? The rock sandstone does not have a crystalline texture. Sandstone has a clastic texture. If you look closely at a sandstone you will be able to see the individual sand grains that are held together. These grains are held together by cement. Cement precipitates from water between sand grains and fills the void spaces between grains. The three most common cements in nature are composed of calcite,silica, and iron oxide. To distinguish these cements, you could drop some dilute hydrochloric acid to see how it behaves. If the sample effervesces rapidly, the cement is calcite. If the cement between grains appears red, the cement is iron oxide. Silica cement is composed of quartz, which is very durable and chemically resistant. 

Clastic texture of the rock sandstone shown in hand sample and under the microscope
Figure 3. Clastic texture of the rock sandstone shown in hand sample and under the microscope. The sketch shows how the grains could be stuck together by cement.

Thinking back to the sandstone example, what is the composition of the sand grains? Following the simple definition of a rock, the sand grains should be minerals. In fact, the grains could be mineral crystals, mineral fragments, rocks, rock fragments, and/or any combination thereof. The label "sand" simply refers to the size of the grains and fragments, not their composition. Assuming you do not have access to a microscope, a rock type that better shows the variation in the grains would be a conglomerate, where the grain size is larger than sand. Looking at the photograph of the conglomerate below, that are many grains or clasts composed of minerals and rocks that are stuck together by a cement. In order to identify whether one of the fragments is composed of granite you would have to identify the minerals within the fragment. Interestingly, if the grains or clasts were not stuck together by cement then it would not be considered a rock. Thus, a pile of sand with unattached sand grains in a pile is not a rock. The same sand attached together using a cement would be considered a rock. 

Photographs of a rock called a conglomerate.
Figure 4. Photographs of a rock called a conglomerate. Notice the variety of rounded grains or clasts composed of rocks and minerals that are joined by a cementing agent. Mineral and rock identification is easier along a surface cut by a rock saw rather than by a naturally weathered surface.
The definition of a rock still holds true: A rock is a consolidated mixture of minerals. Even if grains in a sandstone or conglomerate are composed of rocks, these rocks are composed of minerals. However, there are two types of rock that are not aggregates of minerals, but are instead a mass of glass and organic material. Volcanic glass, or obsidian, is considered a rock. This rock has cooled so quickly that there was no time for a crystalline structure to form. Obsidian’s diagnostic property is its conchoidal fracture pattern that is used to identify this unique rock. Coal is also considered a rock. Coal is plant material, typically from a swamp, that has been compressed through geologic time. This valuable energy source is often identified in hand samples by its vitreous lustre and relatively low specific gravity.
Photographs of unique rocks called a) obsidian and b) coal
Figure 5. Photographs of unique rocks called a) obsidian and b) coal. Notice conchoidal fracture on the obsidian.
What if we discover fossils inside a hand sample that we are trying to identify?  You will find that the original composition of the fossilized organisms have not been preserved, but have been replaced with other minerals similar to the minerals that make up the rest of the hand sample.  If the hand sample was made of limestone, for example, both the rock hand sample and the fossils located within it would be made of the mineral calcite.
Photograph of a rock called limestone with visible fossils called crinoid
Figure 6. Photograph of a rock called limestone with visible fossils called crinoids. This rock is composed of the mineral calcite (CaCO3).
One more consideration that we need to add to our working definition of a rock is that a rock has to be naturally occurring. Any human-made item cannot be considered a rock. For example, a brick or cinder block are not considered rocks because they are not naturally occurring. Interestingly, sometimes a piece of concrete created by humans can look very similar to a conglomerate created by natural processes. 

 

Photographs of a brick and a cinder block both created by humans and therefore not considered a rock.
Figure 7. Photographs of a brick and a cinder block both created by humans and therefore not considered a rock.

In summary, a rock is a coherent, naturally occurring solid, consisting of an aggregate of minerals (or a mass of glass for obsidian or organic material for coal). A mineral is a naturally occurring, inorganic solid with an orderly internal structure and definite chemical composition. Therefore, both a mineral and a rock are naturally occurring solids. They differ in organic composition, and are related in that rocks contain minerals but minerals do not contain rocks. In the next module, we will turn our attention to the classification of rocks and the three main rock types.