Rocks and minerals contain valuable clues about how the Earth formed and developed over billions of years – but what’s the difference between the two?
Essentially, rocks are aggregates of two or more minerals. Minerals, meanwhile, are solids that, with a few exceptions such as diamonds, contain no carbon and are arranged in an orderly, repeating “crystal structure.”
“Minerals are actually the building blocks of rocks,” Erika Andersonan honorary curator of mineralogy and petrology at the Hunterian Museum of the University of Glasgow in Scotland told LiveScience. “It’s a bit like atoms in a molecule, so minerals are the atoms.”
Each type of mineral has a unique crystal structure, which results from its chemical composition and dictates a range of physical properties such as hardness, color or magnetism, depending on the Geological Survey of the United States (USGS). For example, halite – the natural form of sodium chloride (NaCl), from which table salt is made – is a soft mineral that forms clear, cubic crystal fragments. Different minerals, such as aragonite (CaCO3) and calcite (CaCO3), may have the same chemical composition, but their crystal structure and physical properties differ because of the way they are formed.
“For any mineral, they’re going to have a set way in which those atoms bond together,” Anderson said. “Some minerals contain the exact same elements, but they are bonded differently, so it makes them different minerals.”
Related: Fountains of diamonds erupting from the center of the Earth reveal the lost history of supercontinents
A good example of a mineral is quartz, which is found around the world and in various rocks such as granite and quartzite, Anderson said. Quartz is made of the chemical elements silicon and oxygen and has the chemical formula (SiO2). The mineral is colorless in its pure formbut impurities can make quartz crystals look opaque or turn them pink, purple, yellow or brown.
As of May 2024, the International Mineralogical Association – the scientific body responsible for identifying, approving and naming minerals – has listed 6,050 mineral species. Experts distinguish minerals based on their crystal structure, the specific way their atoms or elements are arranged.
While some minerals, such as halite, have relatively simple crystal structures, others may contain 10 or more elements, such as chomyakovite And georgbarsanovite.
“We are consistent find new mineralsbecause we’re exploring areas that could have that circumstances we knew nothing about” said Anderson.
The rock cycle
There are three main types of rocks – igneous, sedimentary and metamorphic rocks – with varying mineral mixtures depending on where and how the rock formed.
Igneous rocks – which form when magma solidifies deep within the Earth or at the surface after a volcanic eruption, for example – contain a limited number of minerals that crystallize. Richard Bevins, an honorary professor of earth sciences at Aberystwyth University in the United Kingdom, told LiveScience in an email. “These are called the common rock-forming minerals and include feldspar, olivine, pyroxene, mica, quartz and amphibole.”
Igneous rocks can be exposed to high heat and pressure, or exposed to fluids that change their mineral composition. Once their mineral composition changes, the rocks are considered metamorphosed, with examples including phyllite, schist, quartzite and marble. Igneous and metamorphic rocks on the Earth’s surface inevitably erode and disintegrate when wind and water impact them. The fragments are transported and form deposits that solidify into new rocks called sedimentary rocks. “Sedimentary rocks are mainly made up of minerals present in the rocks that have been eroded to form the sediment,” Bevins said.
Generally the process by which rocks are formed constantly recycled and transformed by geological processes is known as the rock cycle.
Some rocks are monomineral, meaning they contain only one mineral. Limestone, for example, is a sedimentary rock consisting exclusively of the mineral calcite (CaCO3). So is glacier ice a type of rock consisting of water crystals.
In 2014, scientists proposed to a new type of rock from plastic pollution: plastiglomerate. A team has found that plastic litter on a beach in Hawaii, natural sediments had melted and glued together, creating rocky chunks. And like rocks, researchers said these plastiglomerates could remain in the geological record forever, marking the fragment of Earth’s history that we inhabit.
Editor’s note: This story was updated at 10:30 a.m. EDT to note that most, but not all, minerals do not contain carbon (diamond is a notable exception). It was also updated to note that crystal structure dictates a mineral’s physical properties.