Minerals

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The huge majority of the minerals found in nature are in crystal form. In fact, the history of humanity can also be explained as the history of the control of mineral resources – from which we extract metals, fertilizers, raw materials for hi-tech products, gems, and salt. But they also play a central role in the control of CO2 in the atmosphere or in the generation of earthquakes.

Have you ever thought about how minerals are formed? Do you know how many species of minerals exist? Would you like to know what the largest crystals in the world are and where they are found?

The solar system formed from a nebula of dust and gases that created an incandescent sphere, the sun, and subsequently the remnants of that protosolar nebula condensed into the different planets of the system. The Earth formed around 4.5 billion years ago.

Since then the extraordinary internal energy of the planet has been dying down, creating minerals. 4.42 billion years ago the crust of the planet was created, formed fundamentally by minerals characteristic of volcanic rocks such as olivine, a silicate of magnesium and iron, and the pyroxenes, other silicates of diverse cations. Later, around 4.40 billion years ago, water began to condense on the planet and new minerals were created, such as some clays, quartz and the sulphurs. The progressive oxygenation of the planet bequeathed new minerals such as the sulphates and carbonates, and an important change in mineralogical diversity took place 2.5 billion years ago with what is called the Great Oxygenation Event, an event that is believed to be related to the expansion of photosynthetic organisms.

rId6Today almost five thousand mineral species are known. The vast majority of these are located on the Earth’s surface, of which the silicates represent almost 90%.

The vast majority of minerals are found mixed in polycrystalline rocks. The crystals of rocks are rarely larger than a millimetre in size, and having been formed simultaneously out of a brine, a vapour or gas or molten lava, crashed into each other during their growth, and so their shape is irregular and not very attractive. However, in order to study them under the microscope, scientists prepare very thin sheets a few microns thick to make them transparent to visible light and to be able to observe them under the microscope. When these sheets are studied with polarisers the colours that are formed are the purest possible and the beauty of these photographs is unique. One of the best microscopists of minerals is Professor Bernardo Cesare of the University of Padua.

Rarely have mineral crystals grown freely to show their symmetry in all its splendour. But when they have, their beauty is unquestionable. Whether they are mineral crystals considered to be gems or a simple quartz crystal from the Alps, like this one from the Peabody Museum collection at Yale University (USA), or a pyrolusite dendrite like this one.

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And very rarely, nature gives us truly impressive examples of crystallization such as the case of the giant crystals of gypsum. The Naica cave is an exceptional example of the harmonic beauty of crystalline symmetry.

Did you know that…

  • Scientists have discovered some 3000 different minerals. The silicates, including quartz, mica, olivine and some precious stones like emeralds, are the most abundant type of mineral, as well as being the largest component of rocks. The remaining types include oxides, sulphurs, sulphates, carbonates and halides.
  • The Earth’s crust constitutes only 0.4% of its mass. It is made up mainly of oxygen, magnesium, aluminium, silicon, calcium, sodium, potassium and iron. These eight elements constitute 99% of the minerals in the Earth’s crust. The continents have a thickness of about 35 km and the depth of the oceans only around 7 km.
  • The mantle is far vaster than the crust, with a thickness of about 2900km and making up 68.1% of the Earth’s mass. It consists of silicon, oxygen, aluminium, magnesium, and iron, mostly in the form of olivine.
  • The nucleus of the Earth consists of iron and nickel and makes up 31.5% of its mass.
  • At the current level of consumption, during its life a newborn will need 387 kg of lead, 352 kg of zinc, 598 kg of copper, 14959 kg of iron, 10169 kg of clays, 14080 kg of salt, 775643 kg of stone, sand and gravel as well as 33110 kg of cement.
  • The cooling process of magma determines the size of the mineral crystals formed. When magma cools slowly, large crystals are formed. When magma cools more quickly, smaller crystals are formed.
  • The same happens when crystals are formed from solutions. The largest known crystals formed from solution are the Giant Gypsum Crystals of Naica (Mexico) that reach up to 11 metres long.
  • A geode is a hollow rock, generally rounded with the hollow cavity partially or totally filled with crystals. It is formed out of a bubble in a volcanic rock or a hollow in a sedimentary rock. Eventually the external layer hardens and, over thousands of years, minerals start to precipitate in the hollow from the subterranean water that fills it.

To find out more

  • Wikipedia has a truly excellent entry on minerals (http://en.wikipedia.org/wiki/Mineral), which we recommend as the best place to start learning about the world of minerals.
  • The National Museum of Natural History of the USA has some interesting pages for an introduction to concepts like the structure, morphology and growth of crystalline minerals.

 

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