Geochemistry

Geochemistry is the science that uses the tools and ideas of chemistry to explain how big parts of our world work, like the Earth's crust and its oceans. By studying chemicals in rocks, water, and soil, scientists can learn many interesting things about our planet.

Geochemistry doesn’t just stay on Earth—it looks at the whole Solar System. This helps us understand many important processes. For example, it explains how the hot layers inside the Earth move, how planets form, and why we have different kinds of rocks like granite and basalt.

Geochemistry is a special mix of chemistry and geology. It helps scientists learn about the history of our planet and other worlds in space. This field shows how everything is connected, from the tiniest atoms to the biggest mountains.

History

The word geochemistry was first used by a scientist named Christian Friedrich Schönbein in 1838. At that time, most people used the term "chemical geology" instead.

Geochemistry became its own area of study when labs, like the United States Geological Survey, started in 1884. They began studying the chemicals in rocks and minerals.

Scientists also studied meteorites and compared them to rocks on Earth. This helped us learn more about how Earth and the Solar System formed.

Later, scientists used special tools to look at the structure of crystals. This helped them understand how elements group together in minerals.

Research continues to explore the chemistry of very old Earth and signs of early life.

Subfields

Geochemistry has several important areas of study. One area, called aqueous geochemistry, looks at how elements like copper, sulfur, and mercury move through water and the air.

Other areas include biogeochemistry, which studies how living things change the Earth's chemistry, and cosmochemistry, which studies elements and their forms throughout the cosmos. There is also isotope geochemistry, which looks at different versions of elements on Earth, and organic geochemistry, which studies materials from living things. Photogeochemistry examines how light causes chemical changes on Earth, and regional geochemistry applies these studies to help understand the environment and find minerals.

Chemical elements

The building blocks of materials are called chemical elements. Each element has a special number called its atomic number, which tells how many protons are in its nucleus.

Some elements can have different versions called isotopes. These have the same number of protons but different numbers of neutrons.

Elements are grouped based on how they behave. One way to group them is the Goldschmidt classification. Some elements, like Na and Si, love to combine with oxygen and are found in the Earth's crust. Others, like Fe, prefer to stay close to iron and are found in the core. There are also elements that form sulfides and ones that are most common in the air.

Differentiation and mixing

The Earth and other planets have different materials because of two opposite processes: differentiation and mixing. In the Earth's mantle, differentiation happens at mid-ocean ridges through partial melting. This process leaves more stubborn materials at the base of the lithosphere, while the rest rises to form basalt. When an oceanic plate moves down into the mantle, convection eventually mixes these parts together again. Erosion helps separate granite into clay on the ocean floor, sandstone near continents, and dissolved minerals in ocean water. Processes like metamorphism and anatexis can then mix these materials once more.

A big reason for differentiation is called fractionation, which is when elements and isotopes are spread out unevenly. This can happen because of chemical reactions, changes in state, movement effects, or radioactivity. For example, planets naturally separate into different chemical areas, like iron-rich cores and silicate-rich outer layers.

Isotopes can also separate in mass-dependent and mass-independent ways. Heavier isotopes are more stable and prefer certain chemical states or heavier phases. Scientists compare isotope ratios to a standard to measure these differences. For example, sulfur has stable isotopes like ³²S and ³⁴S, and the ratio between them helps scientists understand processes on Earth.

Cycles

Main article: Geochemical cycle

See also: Climate model § Box models

Chemical elements move around the Earth through processes called geochemical cycles. These cycles change how much of each element is found in different places.

Scientists study these changes using observations and models.

To understand these cycles, scientists group parts of the Earth into areas called geochemical reservoirs. For example, the ocean can be one reservoir or split into several. In simple models, each reservoir is like a box with things going in and out. These models help scientists learn how elements move and change over time.

Abundance of elements

Main article: Abundance of the chemical elements

The Solar System formed from a cloud of gas and dust. The Sun, which is most of the Solar System, is mostly hydrogen and helium. Other elements make up only a tiny part.

Hydrogen and helium were created after the Big Bang. Other elements were made inside stars. Meteorites, pieces of asteroids, help scientists learn about the early Solar System. The giant planets—Jupiter, Saturn, Uranus, and Neptune—are mostly hydrogen and helium. The smaller, rocky planets—Mercury, Venus, Earth, and Mars—have less of these lighter elements. Scientists study these planets using spacecraft and telescopes.

Earth's crust

See also: Abundance of elements in Earth's crust

Most rocks in the Earth's crust are made of oxides, with small amounts of chlorides, sulfides, and fluorides. By 1911, scientist F. W. Clarke found that about 47% of the Earth's crust is oxygen, mostly in compounds called oxides. Important oxides include silica, alumina, iron oxides, and carbonates like calcium carbonate.

These oxides mix in different ways. For example, potash and soda can form feldspars. Phosphoric acid and lime create apatite, and titanium dioxide with ferrous oxide makes ilmenite. When there is extra silica, it becomes quartz, and extra alumina turns into corundum. By studying rocks, scientists can guess what minerals they contain, but there are many exceptions.

Earth's crust is mostly made of silicate minerals, with plagioclase feldspar being the most common, followed by alkali feldspar, quartz, pyroxene, amphiboles, micas, and clay minerals. Non-silicate minerals are a smaller part of the crust.

Rocks with lots of silica are called felsic rocks and often contain quartz. Rocks with very little silica are called mafic rocks and usually have olivine instead of quartz. Intermediate rocks have neither quartz nor olivine. Some special rocks rich in soda are called alkali rocks.

Most rocks contain feldspars or similar minerals. In felsic rocks, common feldspars include orthoclase and microcline, while mafic rocks often have labradorite and anorthite. Augite is common in mafic rocks, but biotite and hornblende are more frequent in felsic rocks.

Trace metals in the ocean

Trace metals mix with important parts of the ocean, like hydroxide, carbonate, and chloride. Their behavior changes depending on whether the water has oxygen or not. Some metals form strong bonds with special molecules, called chelators, which help keep metals in the water instead of solids.

Different metals in the ocean tell us about past conditions. For example, higher amounts of cadmium in ocean sediments might mean that in the past, the ocean had lower oxygen levels. Metals like copper, molybdenum, and others change forms based on the amount of oxygen. In the ocean, metals can spread out in different ways. Some, like molybdenum, stay mostly the same throughout the water. Others, like zinc, are used up by tiny sea plants and then release back into deeper water. Metals such as aluminium attach quickly to particles and are found near the bottom or close to underwater hot springs. Iron and copper behave in mixed ways, influenced by both recycling and attachment to particles. Near hot springs, iron can be found in much higher amounts than in open ocean water.