By Ron B. Davis Jr., Georgetown University
Victor Goldschmidt was a Norwegian geochemist. He had begun his career in mineralogy research in 1914, and by 1937 he was a seasoned researcher in the field. He had taken an interest in the phase behavior of elements and minerals in the Earth, and had set about cataloging the kinds of environments in which elements were most commonly found.
Iron, Silica, and Sulfur Teams
From the atoms that found their way into our solar nebula, a wide variety became a part of our very early Earth, when it was a hot, molten ball. Those were the conditions where Earth’s most common elements, iron, silicon and oxygen, and sulfur, tended to separate the elements into different regions; slowly sinking iron and slowly rising silica and sulfur carried along with them other elements that mixed well into each substance.
With iron, silica, and sulfur separating out, all of the other elements had to make a choice. They had to choose to join ‘Team Iron’ or ‘Team Silica’ or ‘Team Sulfur’. This phenomenon was of great interest to Norwegian geochemist Victor Goldschmidt. He uncovered a striking trend that today’s periodic table highlights.
How the Goldschmidt Classification Works
Goldschmidt created four classifications for the elements based on their associations with iron, silica, and sulfur, the three main environments available to them in the early Earth.
Lithophiles are rock loving or silica loving elements. They are elements like sodium and potassium; those elements that are easily incorporated into the silicate structures produced by silicon and oxygen. As massive amounts of silica migrated upward to the surface, sodium, potassium, and other elements that do not interact favorably with iron, made their way upward along with the silica. This brought to the surface large quantities of useful elements, like aluminum, neodymium, and even uranium.
Siderophiles are iron loving elements. They are dense metallic elements that can bond and interact strongly with iron. Examples of these include platinum and iridium. As the Earth’s huge quantities of iron sank to the core, the iron took along with other elements that interact favorably with the iron. These elements are scarce at the surface of the planet. Yet they’re thought to be very common deep within the core where humans cannot reach them.
Chalcophiles literally mean sulphur loving elements. They include elements like silver and copper, which are easily combined with sulfur and other group 16 elements to form ionic compounds. The smaller amounts of sulfur were enough to help pull up the ‘sulfur-loving’ elements like copper, silver, zinc, and cadmium, and depositing them near the surface.
Finally, there are atmophiles—hydrogen, carbon, nitrogen and most notably the noble gases. Atmophiles literally means vapor loving elements. These elements prefer to stay in the gas phase, which leaves the vast majority of these elements to escape into the atmosphere, and in some cases, even completely dissipate into space. Atmophiles are the spectators in the game. They tend to be lighter elements which combined in the early atmosphere to make water, methane, nitrogen gas, and ammonia.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Elements in Oceans
However, Goldschmidt’s classification system leaves out one major reservoir for elements on the Earth—another important place where metals, metalloids, and nonmetals have come together later during Earth’s history.
The oceans were a late arrival, only forming about 3.8 billion years ago, as the temperature of the surface dropped below 100° Celsius. But 3.8 billion years is a lot of time for the world’s oceans to accumulate dissolved elements.
Water is an example of why hydrogen, although an atmophile, is still the eighth or tenth most common element on Earth. Even though pure hydrogen is too light to be held by Earth’s gravity, hydrogen combines chemically with oxygen to form a very stable water molecule. That fills the world’s ocean basins, rivers, and lakes.
But even the oceans are not pure water. In fact, about 3% of the world’s oceans by mass are a collection of dissolved ions—atoms that have released or accepted electrons to achieve an octet, making them more soluble in water.
Metals in Seawater
Metals are commonly found in seawater, and among the more common metals are sodium, potassium, magnesium, and calcium—lithophilic elements found often in the minerals forming rocks on land. Indeed, most of those ions in the oceans came directly from the weathering of rocks on land through the process of erosion, which washes them into the sea.
Even potentially valuable metals like silver and gold can be detected in everyday seawater—though just as on land, they are present in much smaller quantities.
But the table predicts that metals like sodium, potassium, magnesium, and calcium should all form ions with a positive charge, losing one or two electrons to form an ion with a full valence shell.
In silicate minerals, this charge is balanced by some of the oxygen atoms that lack a second bond to silicon. But in water, this would present a problem since an ocean full of nothing but positively-charged ions would literally be a lightning rod—carrying an overall positive charge that would attract electrons to it.
But a nonmetal element helps to avoid this scenario—chlorine. From group 17, chlorine is a nonmetal element that would prefer to accept an electron, which makes it become a negatively-charged chloride ion. And negatively charged chloride ions help to balance the charge associated with the metals dissolved in the ocean.
In fact, chloride ions are extremely abundant in seawater as they help to provide charge balance for the dissolved metals. It is this sea of ions that gives seawater its taste and part of its interesting chemistry.
Common Questions about the Goldschmidt Classification of the Elements
In the Goldschmidt classification, there’s a class of elements called lithophiles. These are also called silicon-loving elements since they are more commonly found in silicate structures. Uranium, aluminum, potassium, and sodium are examples of lithophilic elements, which can be found on the Earth’s surface.
Siderophiles are elements capable of bonding strongly with iron. During the formation of the earth, iron moved toward the core, carrying large amounts of siderophiles elements such as platinum and iridium deep into the earth.
Chalcophiles are elements that can form ionic bondings with sulfur. These sulfur-loving elements include copper, silver, zinc, and cadmium, which can be found mostly near the Earth’s surface.