Genesis of Minerals - A brief introduction


Minerals - these wonderful creations of nature - how do they come into being?

There are so many different kinds of minerals that have many different colors and crystallographic appearances. Quite often, one single mineral can occur in different shapes. Even the sizes in which we find minerals can vary greatly: You probably know that there a very small and tiny crystals being only a fraction of a milli-meter small (often called mirco-mounts, very nice for microscopy), but did you know that there crystals that exceed 10 meters and more? On this page we will provide short introduction into the genesis of mineral specimens. This page is intended for non-experts and beginners. It should be considered as "permanently under construction". 

What are minerals: Minerals are composed of different elements that occur in nature. To form a mineral the chemical elements have to self-organize themselves into a certain order, the crystal lattice (more on this topic can be found in the Properties Section). Before continuing, it is important to define what a mineral actually is. When speaking of minerals we refer to matter of homogenous chemical composition (this does not need to be a crystal). In contrast to minerals, rocks are macroscopically hetergeneous, i.e. they are composed of different minerals, each of which has a different chemical composition (it shall be noted that the grain size can vary tremendously, though, making it sometimes difficult to see the heterogeneity with the naked eye).

Example: Granite (Granit) is a rock that can form mountains. If you look at a piece of Granite is appears sprinkled - there are light spots and darks spots; depending on the particular piece you are looking at, the "spots" can be very small or rather big. The spots correspond to the individual minerals that make up Granite: Feldspar (Feldspat), Quartz (Quarz), and Mica (Glimmer). Each of these minerals has a different chemical composition, which does not vary within the individual mineral. At the contact of two minerals the chemical composition changes instantaneously. The grain size, i.e. the size of the individual mineral crystals of which the rock is composed, can tell us somthing about the genesis of the rock, such as temperature, pressure, or cooling velocity, all of which may impact crystal growth.

Mineral Genesis I: Having said that minerals are an organized assembly of chemical elements, which are arranged in groups in the periodic table of elements, let us briefly talk about atoms, elements, and molecules: An atom is a tiny particle (in the order of a tenth of the millionth part of a milli-meter, ~ 10E-10 m) consisting of a nucleus that is surrounded by a cloud of electrons, which are negatively charged. The nucleus itself is composed of two kinds of particles, the neutron and the proton, the latter of which carries a positive charge. In an atom, the number of protons in the nucleus is matched by the number of electrons. Thus, an atom is electrically neutral. A particular combination of neutrons, protons, and electrons is called a chemical element. Each element is characterized by a different number of protons and electrons (remember: number of protons = number of electrons in an atom). Important elements are for example oxygen (O), silicium (Si), carbon (C), phosphorous (P), sulfur (S), and hydrogen (H). Atoms can be combined to chemical groups, called molecules. Under certain circumstances, an electron can be stripped away from the atom / molecule or added to its electron cloud, creating a charged particle called an ion. Ions with a positive charge (electrons are lost) are called cations (e.g. lithium ion (Li+),
sodium ion (Na+), potassium ion (K+), or calcium ion (Ca2+)). When charged negatively they are called anions (e.g. oxygen ion (O2-), silicate ion (SiO4(4-)), sulfate (SO4(2-)), phosphate (PO4(3-)), or hydroxyl ion (OH-)).

Mineral Genesis II: Now that we know what atoms, elements, molecules, and ions are, we can continue to ask who these elements find together in nature to form crystals. Well, they all come from the inner part of our earth, either as molten masses or as hot solutions. The inner part of the earth is composed of a hot nickel-iron core, which due to the enormous pressure is solid despite temperatures exceeding the melting point of these metals. The outer core, which is also made from hot molten nickle and iron (NiFe), is liquid. The core, which is ~ 3500 km thick, is surrounded by the ~ 2900 km thick. Near the surface the mantle is a hot, plastic layer called Asthenosphere (~ 100 km). The Asthenosphere is followed by the outermost part of the earth, the crust, which itself can be divided into the Oceanic Crust and the Continental Crust. The crust is solid and cold and just perfect for us to life on it. The crust is not a single surface. Rather, it is divided into plates, which float on the molten part of the mantle.

Scheme of EarthScheme of the layers of Earth

Mineral Genesis III - Plate Tectonics: Now, the plates of earth´s crust behave like cars in a motodrome - floating on molten rock they bump into each other. When two plates pump into each other, usually one plate slides under the other. Zones where this happens are called Subduction Zones and often are zones where earthquakes happen. And: these are zones, where minerals are formed, where they are born - so to say. This can actually happen by different ways:

Scheme of Plate Tectonics

Volcanoes are often found in subductions zones. In a volcano, molten rock from the inner part of the earth (called Magma) is erupted. After eruption the molten rock is called Lava. Well, lava itself does not contain a lot of minerals (well, of course this is not true, but let´s assume it for now). But magma does not necessarily have to reach the surface. It can also be injected into solid rock (which actually is not really solid, but more jelly-like in the depth of the earth) in the depth forming kind of magma-bubbles. In such a bubble complicated chemical processes can take place. For instance, magma has a certain chemical composition, which can be changed by e.g. dissolving some of the host rock the magma was injected into. What happens now can perhaps be compared best to crystallization of salt (sodium chloride NaCl) out of a solution in water:

Crystallization of salt (sodium chloride, NaCl) from water  - growing a crystal: This is an experiment that many of us have performed, either in school or just to get a nice salt crystal. So, what happens: Sodium chloride is a compound formed from the ions Na+ and Cl-. These ions are held together by so called ionic forces. Water, on the other hand, is a very polar solvent that can solvate ions very well. If a crystal of sodium chloride is put into water the water molecules "break" ions out of the crystal´s lattice. This way, the crystal is dissolved step by step. Water has the property to be able to dissolve more salt when the temperature is higher. Now, if you want to grow a nice salt crystal, you heat a cup of water and put salt into the hot solution until you cannot dissolve any more. The solution is now called saturated. So far, so good. What happens to the solution when we start cooling? We just learned that water can dissolve more salt when its hot and less when being cold. When we now start to cool down our water solution, we have more ions dissolved (left from higher temperature) than the solution can handle at this temperature. The excess ions now start to reassemble into crystals. Depending on how fast we drop the temperature we can control if either many small crystals (or more nuclei for crystallization) or only few crystals come into existence. This will normally also decide whether we get large or small crystals: usually, few seeds (crystallization nuclei) can grow rather large crystals, while many seeds usually will result in a large number of small crystals.

So, we have a hot melt with all the ingredients to form crystals. This melt, by the way can be of very low viscosity (that is: how easily a fluid can flow. Honey, for example, is very viscous compared to water) due to the presence of volatile elements and water. Now, when this melt is injected into the surrounding host rock of lower temperature, the solution will start to cool and crystals will grow. Also, crystal-growth can be induced by a drop in pressure. To complicate things further, we have a very complex composition of the solution. When crystallization starts in a pegmatitic pocket, usually a number of different minerals will grow crystals. These minerals will not all start growing crystals at the same temperature at the same time. In more detail: When cooling starts, the first minerals will grow crystals thereby changing the solutions composition thereby producing a new melt composition reasdy to grow crystals of a new mineral. This procedure is called fractionated crystallization. The consequence of this process is that e.g. pegmatitic minerals, crystallize in a certain order. This explains, for example, a tourmaline inside a quartz crystal - the quartz simply grew later than the tourmaline.

Intrusion of Pegmetite into Host Rock    Typical Pocket in Pegmetites

Analogy: Fractionated Crystallization can be viewed as analogous process to distillation - a process that makes separates e.g. methanol out of whisky brew. When a mixture of water, methanol (which makes one blind and finally kills one if drunk), and ethanol is slowly heated, the following happens: The temperature rises until the boiling point of methanol (the lowest boiling solvent in this mixture) - and methanol goes from the liquid into the gas phase. Despite we continue to supply energy the temperature does not rise until all of the methanol is distilled off. During this process the composition of the solution continuously changes. Then, when the entire methanol has been removed from the solution, the temperature rises to the boiling point of ethanol and stays again at this temperature until the ethanol is completely removed. The solvent composition changes again, until we are left with pure water. During crystallization we remove energy from the system by cooling, and we make a phase transition from the liquid to the solid phase.

So, pegmatites form in the depth of earth, crystals form in a complex process during cooling and pressure release. During crystallization the composition of the melt changes, some crystals may actually form and be dissolved again later during crystal formation. Some crystals show zoned growth, meaning that some minerals start growing under a certain melt composition, and continue to grow with a different chemical composition, which may lead to changes in color etc. Fractionated crystal growth can also explain inter-grown minerals, e.g. a tourmaline inside a quartz crystal - the quartz grew at a later stage and grew around the tourmaline.

An important question is now: when crystals are formed kilometers below the surface of the earth, how can we collect them? Plate tectonics helps us in this case. When plates collide, one plate is pushed under the other, while the other plate is lifted and piled up. This way pegmatites formed in the depth are lifted up. Still, their are surrounded by host rock. However, over time, water, heat and freezing, and wind cause degradation of rocks. This process is called weathering. Plate tectonics and weathering are actually the two players determining growth of formation of mountains, such as the Alps or the Himalaya: while plate tectonics folds and lifts the rocks, weathering decomposes it. Depending which process is stronger, we can observe growing or shrinking mountains. And weathering decomposes the host rock so that we can now find exposures of pegmatites at the surface. When a pegmatite is found to bear gems the pegmatite is then often systematically mined, often by putting underground tunnels in the earth following the pegmatite seam, which is often only a meter of less in thickness.

Fter talking so much about pegmatites, we have finally a few pictures here: from left to the reight you can see a pegmatit vein cutting through the darker host rock. The pegmatite is lighter in color, but is not unifomly structured. Near the contact one can see tiny black spots, which are black tourmalines. At the lower contact the rock looks sparkled again, which is called graphic granit. The picture in the middle shows an excavated pocket in the same pegmatite, which is about 1 m in thinckness. Finally, on the right you can see a tourmaline in place how it was found in the pocket.

Pegmatite vein cuttin through host rockPegmatit PocketGem in Pocket

Pegmatology, the science investigation the properties of pegmatites, is a very complicated field. Of the many pepgmatites out there only few are actually producing minerals. Very important is the chemical composition, in particular the presence of rare elements, such as lithium, caesium, and tantalium form the so called LCT-type pegmatites. If you would like to learn more about pegmetites, we would like to refer you to a text book on pegmatites, which you can hopefully find in your local library. However, minerals are also formed by other processes.

For example, by so-called hydrothermal growth. During uplift and folding of rocks fractures occur. The fractures can be filled with hot aqueous solutions. Water that penetrated into the ground from the surface can be heated in the depth of earth in zones with geological activity. The hot water - in combination with pressure - can dissolve rocks or at least extract certain chemicals from surrounding rocks. This situation has already been described above, when we dissolved salt in hot water. The salt started to crystallize when the solution cooled. The same happens when hot aqueous solutions start to cool - only the chemical composition is much more complex. Again we end up with fractionated crystallization, this time from a hot solution instead from a melt. In fact, both ways can produce the same minerals. Also, pegmatites can be hydrothermally altered meaning that hot solutions penetrate a pegmatite after formation.

Exposure of a hydrothermal pocket by weathering

Minerals can also be formed by a process called metamorphosis. During metamorphosis minerals that have already been formed are altered. For example: marble. Marble is made from limestone. Limestone is made from smallest marine animals that lived in prehistoric oceans. When these animals died their shells sank on the ground of the ocean and formed thick layers there. In the course of plate tectonics these layers became pressurized, folded, and up-lifted to form mountains. When Limestone is put under a certain pressure in combination with a particular temperature, the mineral is altered. In this particular case the chemical composition remains the same. What is changed is the crystal-structure, the way the molecules are arranged. And this is what gives marble its special look. However, you should not try to clean your marble tiles with acidic cleaning agents - as limestone, it is dissolved by acids.

There are even more ways how minerals can be generated. For example, water soluble minerals can be dissolved at the place where the were generated in the first place and re-crystallized at another location. This is called a secondary mineralization or a secondary deposit. Another scenario is that one mineral grows and it later replaced by another. In this case, however, the crystal has the shape of the mineral that was formed first, which is called pseudomorphosis. Gems are often found in the course of a river. However, they are not produced there, the were originally from, e.g., pegmatitic or hydrothermal origin. But weathering exposed the mineral bearing pockets and water washed it into the rivers. This kind of deposit is called a placer.

We hope, we have provided you with a first impression of how minerals are generated. If you are interested in more details, we refer you to textbooks of geology and mineralogy. If you have found a mistake on this page please be so kind and let us know!


Acknowledgement:

We thank Dr. J.E. Patterson (U. Calgary) for providing pictures of pegmatites and for many long and stimulating discussions and for being tireless in teaching us about rocks and pegmatites!