Planet Earth/3g. Common Inorganic Chemical Molecules of Earth

Goldschmidt Classification
With 118 elements on the periodic table of elements, there can be a nearly infinite number of molecules with various combinations of these 118 elements. However, on Earth some elements are very rare, while others are much more common. The distribution of matter, and of the various types of elements across Earth’s surface, oceans, atmosphere, and within its inner rocky core is a fascinating topic. If you were to grind up the entire Earth, what percentage would be made of gold? What percentage made of oxygen? How could one calculate the abundances of the various elements of Earth? Insights into the distribution of elements on Earth came about during World War II, as scientists developed new tools to determine the chemical makeup of materials. One of the great scientists to lead this investigation was Victor Goldschmidt.

On November 26, 1942, Victor Goldschmidt stood among the fearful crowd of people assembled on the pier in Oslo, Norway, waiting for the German ship Donau to transport them to Auschwitz. Goldschmidt had a charmed childhood in his home in Switzerland, and when his family immigrated to Norway, Goldschmidt was quickly recognized for his early scientific interests in geology. In 1914 he began teaching at the local university after successfully defending his thesis on the contact metamorphism in the Kristiania Region of Norway. In 1929 he was invited to Germany to become the chair of mineralogy in Göttingen, and had access to scientific instruments that allowed him to detect trace amounts of elements in rocks and meteorites. He also worked with a large team of fellow scientists in the laboratories whose goal it was to determine the elemental make-up of the wide variety of rocks and minerals. However, in the summer of 1935 a large sign was erected on the campus by the German government that read, “Jews not desired.” Goldschmidt protested, as he was Jewish and felt that the sign was discriminatory and racist. The sign was removed, but only to reappear later in the Summer, and despite his further protest against the sign, the sign remained as ordered by the new Nazi party. Victor Goldschmidt resigned his job in Germany and returned to Norway to continue his research, feeling that any place where people were injured and persecuted only for the sake of their race or religion, was not a welcome place to conduct science. Goldschmidt had with him vast amounts of data regarding the chemical make-up of natural materials found on Earth, particularly rocks and minerals. This data allowed Goldschmidt to classify the elements based on their frequency found on Earth.

The Atmophile Elements
The first group Goldschmidt called the Atmophile elements, as these elements were gases and tended to be found in the atmosphere of Earth. These included both Hydrogen and Helium (the most abundant elements of the solar system), but also Nitrogen, as well as the heavier noble gasses: Neon, Argon, Krypton and Xenon. Goldschmidt believed that Hydrogen and Helium as very light gasses were mostly stripped from the Earth’s early atmosphere, with naturally occurring Helium on Earth found from the decay of radioactive materials deep inside Earth, and trapped, often along with natural gas underground. Nitrogen forms the most common element in the atmosphere, as a paired molecule of N2. It might be surprising that Goldschmidt did not classify oxygen within this group, and that was because oxygen was found to be more abundant within the rocks and minerals he studied, in a group he called Lithophile elements.

The Lithophile Elements
Lithophile elements or rock-loving elements are elements common in crustal rocks found on the surface of continents. They include oxygen and silicon (the most common elements found in silicate minerals, like quartz), but also a wide group of alkali elements belong to this group including lithium, sodium, potassium, beryllium, magnesium, calcium, strontium, as well as the reactive halogens: fluorine, chloride, bromine and iodine, and with some odd-ball middle of the chart elements, aluminum, boron, phosphorous, and of course oxygen and silicon. Lithophile elements also include the Rare Earth elements found within the Lanthanides, and making a rare appearance in many of the minerals and rocks understudy.

The Chalcophile Elements
The next group are the Chalcophile elements or copper-loving elements. These elements are found in many metal ores, and include sulfur, selenium, copper, zinc, tin, bismuth, silver, mercury, lead, cadmium and arsenic. These elements are often associated in ore veins and concentrated with sulfur molecules.

The Siderophile Elements
The next group Goldschmidt described where the Siderophile elements or iron-loving elements, which include iron, as well as cobalt, nickel, manganese, molybdenum, ruthenium, rhodium, palladium, tungsten, rhenium, osmium, iridium, platinum, and gold. These elements were found by Goldschmidt to be more common in meteorites (most especially in iron-meteorites) when compared to rocks found on the surface of the Earth. Furthermore, these elements are common in iron-ore and associated with iron-rich rocks when they are found on Earth’s surface.

The last group of elements are simply the Synthetic elements, or elements that are rarely found in nature, which include the radioactive elements found on the bottom row of the Periodic Table of Elements and produced only in labs.

Meteorites, the Ingredients to Making Earth
A deeper understanding of the Goldschmidt classification of the elements was likely being discussed at the local police station in Oslo on that chilly late November day in 1942. Goldschmidt’s Jewish faith resulted in his imprisonment seven years later, when Nazi Germany invaded Norway, and despite his exodus from Germany, the specter of fascism had caught up with him. Jews were to be imprisoned, and most would face death in the concentration camps scattered over Nazi-occupied Europe. Scientific colleagues argued with the authorities that Goldschmidt’s knowledge of the distribution of valuable elements was much needed. The plea worked, because Victor Goldschmidt was released, and of the 532 passengers that boarded the Donau, only 9 would live to see the end of the war. With help, Goldschmidt fled Norway instead of boarding the ship and would spend the last few years of his life in England writing a textbook, the first of its kind on the geochemistry of the Earth.

As a pioneer in understanding the chemical make-up of the Earth, Goldschmidt inspired the next generation of scientists to study not only the chemical make-up of the atmosphere, ocean, and rocks found on Earth, but to compare those values to extra-terrestrial meteorites that have fallen to Earth from space.

Meteorites can be thought of as the raw ingredients of Earth. Mash enough meteorites together, and you have a planet. However, not all meteorites are the same, some are composed mostly of metal iron, called iron meteorites, other meteorites have equal amounts of iron and silicate crystals, called stony-iron meteorites, while the third major group, the stony meteorites are mostly composed of silicate crystals (SiO2). If the Earth formed from the accretion of thousands of meteorites, then the percentage of chemical elements and molecules found in meteorites would give scientists a starting point for the average abundance of elements found on Earth. Through its history Earth’s composition has likely changed as elements became enriched or depleted in various places, and within various depths inside Earth. Here are the abundances of molecules in meteorites: (From Jarosewich, 1990: Meteoritics)

If Earth was a homogenous planet (one composed of a uniform mix of these elements) the average make-up of Earthly material would have a similar composition to a mix of stony and iron meteorites. We see some indications of this fact, for example SiO2 (silica dioxide) is the most common molecule in stony meteorites at 38.2%, with silica bonded to two oxygen molecules. Silicon and oxygen are the most common molecules found in rocks, forming a group of minerals called silicates, which include quartz, a common mineral found on the surface of Earth. The next three molecules, MgO, FeO, and CaO are also commonly found in rocks on Earth, however, iron (Fe) which is very common in iron meteorites, and also makes up a significant portion of stony meteorites with various molecules containing FeO, FeS, and Fe in native metal form. Yet typical rocks found on the surface of Earth contain very little iron. Where did all this iron go?

Goldschmidt suggested that iron (Fe) is a Siderophile element, as well as nickel (Ni), manganese (Mn) and cobalt (Co), which sank into the core of the Earth during its molten stage. Hence over time the surface of the Earth became depleted in these elements. A further line of evidence for an iron rich core is Earth’s magnetic field observed with a compass. This magnetic field supports the theory of an iron rich core at the center of Earth. Hence siderophile elements can be thought of as elements that are more common in the center of the Earth, than on Earth’s near surface. This is why other rare siderophile elements like gold, platinum and pallidum are considered precious metals at the surface of Earth.

Goldschmidt also looked at elements common in the atmosphere, in the air that we breath and that readily form gasses with Earth’s temperatures and pressures. These atmophile elements include hydrogen and helium, which are only observed in meteorites as H2O and very little isolated helium gas. This is despite the fact that the sun is mostly composed of hydrogen and helium. If you have ever lost a helium balloon, you likely know the reason why there is so little hydrogen and helium on Earth. Both hydrogen and helium are very light elements and can escape into the high atmosphere, and even into space. Much of the solar system’s hydrogen and helium is found in the sun, which has a greater gravitational force, as well as the larger gas giant planets in the outer solar system, like Jupiter which has an atmosphere composed of hydrogen and helium. Like the sun, larger planets can hold onto these light elements with their higher gravitational forces. Earth has lost much of its hydrogen and helium, and almost all of Earth’s hydrogen is bonded to other elements preventing its escape.

Nitrogen is only found in trace amounts in meteorites, as the mineral carlsbergite, which is likely the source of nitrogen in Earth’s atmosphere. Another heavier gas is carbon dioxide (CO2), which accounts for about 0.1% of stony meteorites. However, in the current atmosphere it accounts for less than a 0.04%, and as a total percentage of the entire Earth much less than that. In comparing Earth to Venus and Mars, carbon dioxide is the most abundant molecule in the atmosphere of Venus and Mars, accounting for 95 to 97% of the atmosphere on these planets, while on Earth it is a rare component of the atmosphere. As a heavier molecule than hydrogen and helium, carbon dioxide can stick to planets in Venus and Earth’s size range. It is likely that Earth early in its history had a similar high percentage of carbon dioxide as found on Mars and Venus, however over time it was pulled out of the atmosphere. This process was because of Earth’s unusual high percentage of water (H2O). Notice that water is found in stony meteorites, and this water was released as a gas during Earth’s warmer molten history, and as the Earth cooled, it resulted in rain that formed the vast oceans of water on its surface today. There has been a great debate in science as to why Earth has these vast oceans of water and great ice sheets, while Mars and Venus lack oceans or significantly large amounts of ice. Some scientists suggest that Earth was enriched in water (H2O) from impacts with comets early in its history, but others suggest that enough water (H2O) can be found simply from the molten gasses that are found in rocks and meteorites that formed the early Earth.

So how did this unusual large amount of water result in a decrease of carbon dioxide in Earth’s atmosphere? Looking at a simple set of chemical reactions between carbon dioxide and water, you can understand why.

Note that g stands for gas, l for liquid, and aq as an aqueous solution (dissolved in water), and also notice that this reaction goes in both directions with the double arrows. Each carbon atom takes on an additional oxygen atom, which results in two extra electrons, this results in the ion CO3-2. This ion forms ionic bonds to two hydrogen ions (H+), forming H2CO3. Because these hydrogen ions can break apart from the carbon and oxygen, this molecule in a solution forms a weak acid called carbonic acid. Carbonic acid is what gives soda drinks their fizz. If water falls from the sky as rain, the amount of carbonic acid would cause a further reaction to solid rocks composed of calcium. Remember that calcium forms ions of Ca+2, making these ions ideal for reacting with the ions to form Calcium Carbonate (CaCO3) a solid.

Note that there is a 2 before the ion so that the amount of each element in the chemical reaction is balanced on each side of the chemical reaction.

Over long periods of time the amount of carbon dioxide will decrease from the atmosphere, however, if the Earth is volcanically active and still molten with lava, this carbon dioxide would be re-released into the atmosphere as the solid rock composed of calcium carbonate is heated and melted (a supply 178 kJ of energy will convert 1 mole CaCO3 to CaO and CO2).

This dynamic chemical reaction between carbon dioxide, water and calcium causes parts of the Earth to become enriched or depleted in carbon, but eventually the amount of carbon dioxide in the atmosphere will reach an equilibrium over time, and during the early history of Earth water scrubbed significantly amounts of carbon dioxide out of the atmosphere of Earth.

Returning to the bulk composition of meteorites, oxygen is found in numerous molecules, including some of the most abundant (SiO2, MgO, FeO, CaO). One of the reasons, Goldschmidt did not include oxygen in the atmophile group of elements was because it is more common in rocks, especially bonded covalently with silicon in silica dioxide (SiO2). Pure silica dioxide is the mineral quartz, a very common mineral found on the surface of the Earth. Hence oxygen, along with magnesium, aluminum, and calcium, is a lithophile element. Later we will explore how Earth’s atmosphere became enriched in oxygen, an element much more commonly found within solid crystals and rocks on Earth’s surface.

Isolated carbon (C) is fairly common (0.5%) in meteorites, but carbon bonded to hydrogen CH4 (methane) or in chains of carbon and hydrogen (for example C2H6) are extremely rare in meteorites. A few isolated meteorites contain slightly more carbon (1.82%) including the famous Murchison and Banten stony meteorites which exhibit carbon molecules bonded to hydrogen. Referred to as hydrocarbons, these molecules are important in life, and will play an important role in the origin of life on Earth. But why are these hydrocarbons so rare in meteorites?

This likely has to do with an important concept in chemistry called Enthalpy. Enthalpy is the amount of energy gained or lost in a chemical reaction at a known temperature and pressure. This change in enthalpy is expressed as (ΔH) and expressed in Joules of energy per Mole. A Mole is a unit of measurement that relates the number of atoms per gram of a molecule’s atomic mass. A positive change in enthalpy indicates an endothermic reaction (requiring heat), and while negative change in enthalpy releases heat resulting in an exothermic reaction (producing heat). In the case of hydrocarbon (like CH4) and the presence of oxygen, there is an exothermic reaction, that releases 890.32 kilojoules of energy as heat per mole.

The release of energy via this chemical reaction makes hydrocarbons such a great source for fuels, since they easily react with oxygen to produce heat. In fact, methane or natural gas (CH4) is used to generate electricity, heat homes and used to cook food on a gas stove. This is also why hydrocarbons are rarely found when closely associated with oxygen. Hydrocarbons are however of great importance, not only because of their ability to combust with oxygen in these exothermic reactions, but because they are also the major elements found in living organisms. Other elements that are important for living organisms are phosphorous (P), nitrogen (N), oxygen (O), sulfur (S), sodium (Na), magnesium (Mg), calcium (Ca) and iron (Fe). All of these lithophile elements are found in complex molecules within life forms near the surface of Earth which are collectively called organic molecules, which bond with carbon and hydrogen in complex molecules found within living organisms. The field of chemistry that study these complex chains of hydrocarbon molecules is called organic chemistry.

Goldschmidt’s classification of the elements is a useful way to simplify the numerous elements found on Earth, and way to think about where they are likely to be found, whether in the atmosphere, in the oceans, on the rocky surface, or deep inside Earth’s core.