4Rocks vs. Minerals

What distinguishes a rock from a mineral?



Mineralogy is the study of geochemistry. Petrology is the study of rocks. Rocks are naturally occurring aggregates of minerals. Unlike minerals, rocks do not have definite chemical structure and composition. Rocks are classified by their composition, textures, and the processes that account for their formation. These factors differentiate rocks into three different categories: igneous, sedimentary, and metamorphic.


Igneous (ignis = fire) rock forms as molten rock cools and solidifies into a crystalline solid. The cooling of magma and lava results in the crystallization of minerals and the formation of an igneous rock. Magma that cools and crystallizes below the surface forms intrusive (plutonic) rocks. At the surface lava solidifies and volcanic debris (pyroclastic material) forms extrusive (volcanic) rocks. Where these rocks cool and solidify results in textures that are specific to their formation. 


Sedimentary rocks are typically layered rocks that are derived from the weathering and erosion processes that form sediments. Approximately 75% of exposed land surface is covered by sediments and sedimentary rocks, as well as the majority of the ocean floor. These sediments will eventually be deposited and become lithified (compacted and cemented) to form a new sedimentary rock. 


Sedimentary rocks can be classified into two different groups: detrital and chemical/organic sedimentary rocks. Detrital (clastic) sedimentary rocks have a texture that consists of small rock fragments and particles that are compacted and cemented together. Detrital sedimentary rocks are subdivided primarily by grain size and roundness of the clast particles. Chemical/organic sedimentary rocks are subdivided into groups based on the mineral composition of the rock. 


About half of the world’s oil and gas, much of its groundwater, and extensive deposits of metallic ores are held in carbonates. Carbonate sediments commonly form in shallow warm oceans either by either direct precipitation out of seawater or from the biological extraction of calcium carbonates from seawater in the form of skeletal material. Carbonate rocks are composed of Calcium carbonate (CO3). When carbonates break down they produce carbon dioxide and water (CO2). This reaction commonly occurs when acid is placed on a carbonate rock like limestone. The resulting reaction causes the carbonate to fizz, distinguishing carbonates from detrital sedimentary rocks. The Earth’s crust stores the largest amount of carbon, where it is a part of a variety of different rock types, with limestone being the largest depository.


Metamorphism (to change form) is a process in which a parent rock undergoes changes in the mineralogy, texture, and sometimes the parent rocks chemical composition. Every metamorphic rock forms from a preexisting rock, be it a metamorphic, igneous, or sedimentary rock, called the parent rock. Metamorphism of a parent rock results in metamorphic textures that describe the size, shape and arrangement of mineral crystals of a metamorphosed rock. Metamorphism of rocks results from subjecting the parent rock to heat, confining pressure, directional (differential) stress, and chemically active fluids. Each of these agents of metamorphism may contribute to the degree of metamorphism, but each agent will vary from one environment to another. Heat is the primary agent responsible for metamorphism; heat provides the necessary energy to chemically alter the parent rock that results in the recrystallization of existing minerals. 


Due to the driving forces of the water cycle and plate tectonics, rocks do not remain the same throughout geologic time. Rocks are forced to undergo change as their environments change over time, this process of change is reflected in the rock cycle.


Minerals are very beautiful but also are important to society. It is the mineralogy of the rocks that drives geologists to study particular potential source rocks (typically organic rich shales) and make the determination that the organic material in the source rock can produce hydrocarbons. Shale characteristics important to the oil and gas industry include; where and how much is present, how much organic matter it contains, the type of organic matter (gas- vs. oil-rich shale), clay and other minerals it contains, how deeply it was buried and “cooked”, its brittleness vs. ductility (break or bend), and how fractured it is (natural fractures). All of these characteristics vary in a shale formation across a region, and it’s these characteristics that are unique to Marcellus and Utica shales that have driven these oil plays in Ohio.