The Building Blocks of Our Rocks

What is a mineral and what properties distinguish one mineral from another?



Minerals are the building blocks of rocks; rocks are composed of minerals. Minerals are naturally occurring, inorganic, crystalline structure with a specific chemical composition. Minerals are composed of atoms of one or more elements that combine in such a way to give that mineral its specific chemical structure.


The chemical bonding and composition unique to each mineral will result in specific properties that are individual to each mineral and its chemical structure. These properties are therefore useful when identifying minerals, and include: hardness, color, clarity, streak, crystal form (habit), cleavage, fracture, tenacity, and density. Other useful properties of minerals sometimes include: smell, taste, reaction to hydrochloric acid, magnetism, and double refraction of light.


Minerals can form under a variety of conditions, such as: during the cooling of molten materials (from lavas, igneous rocks), during the evaporation of liquids (salt, gypsum, the cooling of liquids (saturated solution). At high temperature and pressures new crystals may also grow in solid materials (metamorphism).


There are thousands of minerals that have been identified, but only a few are common to the Earth’s crust and these are referred to as rock forming minerals. Silicate minerals dominate the Earth’s crust and form when the two most common elements (silicon and oxygen) in the Earth’s crust combine. The clay minerals, kaolinite, smectite, illite, and chlorite are ever-present in the rocks being targeted for the exploration of hydrocarbons. Studying clay mineralogy of specific source rocks is important since clay minerals and organic matter are responsible for the formation of hydrocarbon generation and typically coexist in the same sedimentary rocks. It is the mineralogy that drives geologists to study potential source rocks (typically organic rich shales) and helps make the determination of the organic materials in the source rocks that can produce hydrocarbons.


The formation of minerals in Ohio over the course of Earth’s geologic past has produced six nonfuel minerals (also called industrial minerals) and coal. Primarily used as construction minerals these industrial minerals include: limestone and dolomite, sand and gravel, sandstone and conglomerate, clay, shale, and salt. Of Ohio’s 88 counties, 87 of those counties have had at least one of these mineral commodities mined. According to the Ohio Department of Natural Resources, of the 564 industrial-mineral-mining operations known to be operating in 2014, 335 operations were reported as active. The economic value of Ohio’s coal and industrial minerals in 2014 totaled nearly $2 billion.