About 90 elements occur naturally on Earth yet most of the rocks beneath our feet, the air that we breathe, and the water that we drink, are composed of just a few of these. Just eight elements account for almost 99% of the atoms in the Earth’s crust, of which just three – oxygen (O), silicon (Si) and aluminium (Al), make up more than 85%! Many of the other elements are very rare and it is only when they become concentrated by geological processes, particularly the effects of heat and water, that it becomes economic to mine them.
Elements of the Sea and Sky
Hydrogen (H) is the most abundant element in the Universe, forged in the Big Bang more than 13.7 billion years ago. Water (H2O) may seem unremarkable but every hydrogen atom in the water we drink, or in our body, was created in the Big Bang.
Oxygen (O), in water and in the air we breathe, was formed in stars that first shone billions of years later – but still long before our own Solar System formed more than 4.5 billion years ago.
Silicon (Si) and oxygen (O) are the two most abundant elements in the Earth’s crust. Combined together as silica (SiO2) they form quartz, the most common mineral at the Earth’s surface. Quartz can form beautiful hexagonal crystals under the right circumstances, and is sometimes coloured by impurities to produce gemstone minerals such as amethyst or rose quartz. However, most quartz is found as irregular veins within other rocks, or has been eroded into pebbles or sand grains.
Peridot and Earth's mantle
Some of the most common elements on Earth combine together to form the mineral peridot, also known as olivine. It is a silicate, a compound in which silicon (Si) and oxygen (O) are combined with other elements, in this case iron (Fe) and/or magnesium (Mg). Peridot is a beautiful olive green colour but it is found mostly deep beneath the Earth’s surface, in the Mantle, and in certain dark-coloured volcanic rocks.
Precious metals from Outer Space
Gold (Au) is one of the rarest elements on Earth. When Earth first formed and melted, iron (Fe), nickel (Ni) and other dense metals – including gold – sunk to form Earth’s core, leaving Earth’s early crust virtually devoid of precious metals. The gold we find in the Earth’s crust today was delivered by meteorites, that contain minute traces of gold, falling in the billions of years since Earth’s surface cooled and solidified. Tectonic and hydrothermal (hot water) processes have since concentrated this gold into deposits that are economical to mine.
The rarest element on Earth
Iridium (Ir) is the rarest stable element in the Earths crust. For every iridium atom there are perhaps seven hundred million silicon (Si) atoms. Iridium is slightly less rare in meteorites and so when a giant meteorite hits Earth it leaves a thin layer of dust relatively enriched in this rare element, providing evidence of the impact.This rock from north-west Scotland was formed by a meteorite impact 1.2 billion years ago. It contains relatively high levels of iridium, in this case 20 parts per billion, but it would still take about 5000 tonnes of this rock to produce a 10g iridium bead, like those at the right hand end of this block!
Fire and brimstone
Sulfur (S), once known as brimstone, commonly is associated with volcanoes, which emit huge volumes of sulfurous gases and commonly deposit solid sulfur around fumaroles, or gas vents.Most elements are combined with other elements in compounds, with sulfur forming many compounds such as sulfides and sulfates, but sulfur is one of the few that is also commonly found in the Earth’s crust in an uncombined, or ‘native’, state.The spelling of sulfur with an ‘f’ rather than a ‘ph’ is now official and accepted by most chemists.
Metal ores: Sulfides
Many elements have an affinity with the element sulfur (S), combining with it to form metal sulfides. These form some of the major metal ores that are mined around the world. This sample is a mix of zinc sulfide (ZnS, the brown mineral) and lead sulfide (PbS, the grey mineral). It is from the Tara Boliden Mine, near Navan in Co. Meath, the largest zinc mine in Europe.
Metal ores: Oxides
Several important metal ores are compounds of the metal with oxygen. Chromite is an oxide of chromium and iron (FeCr2O4) and is the main ore of the metal chromium.The only significant deposits of chromite anywhere in the UK are on the island of Unst, in Shetland. They were mined from the 1870s until the 1920, for pigments and later for steel making, but the deposits are too small to be economic today.This sample was presented to the Elements exhibition by Geopark Shetland.
Metal ores: Aluminium
Aluminium (Al) is the third most abundant element in the Earth’s crust, after oxygen (O) and silicon (Si). Combined with these elements, and others, it forms many common rocks and minerals, but they contain too little of the metal to be worth mining. Humid weathering of these rocks can produce bauxite, a rock rich in aluminium hydroxides and iron oxides (such as Al[OH]3 and Fe2O3). Bauxite was mined in parts of north-east Ireland up until the 1940s and a layer of it can be seen in the cliffs at the Giant’s Causeway.
Manganese nodules and Russian subs
Manganese (Mn) oxide nodules form on the ocean floor and also contain iron(Fe), nickel (Ni) and copper (Cu), but the cost of mining them more than 5 km down in the ocean makes it uneconomic. However, a supposed expedition in 1974 to mine manganese nodules from the sea floor using a purpose-built ship, the Hughes Glomar Explorer, was actually a cover that Howard Hughes and the CIA used while retrieving a sunken Russian submarine from the sea bed.
Salts of the Earth
We usually think of metals as coming from metallic ores, but several of the lightest metal elements, including magnesium (Mg), sodium (Na) and potassium (K), actually are obtained from salts deposited by the evaporation of ancient sea water. These sample jars, donated by the German Potash Syndicate in 1914, are from Stassfurt in central Germany which was a major producer of magnesium metal in the early 20th Century.
Limestone and carbonates
Carbonates are compounds of carbon and oxygen with other elements. Combined with calcium (Ca) it forms the mineral calcite (CaCO3), the rock limestone, and many of the fossils that limestones commonly contain.
Carbonates of other elements can form important ore deposits, such as malachite (copper carbonate, Cu2CO3[OH]2) and smithsonite (zinc carbonate, ZnCO3).
Hydrocarbons, as their name suggests, are compounds of just two elements, hydrogen (H) and carbon (C). The simplest hydrocarbon is methane (CH4), a gas, but the more carbon atoms there are in the molecules, the more viscous the hydrocarbon. Petrol molecules contain about eight carbon atoms (C8H18) while bitumen contains 35 or more (C35H72). Crude oil straight from the ground contains a variety of different hydrocarbon molecules. It was formed by the effect of heat and pressure on organic material, mainly the remains of plankton, trapped in sediments as they accumulated.