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4.3: Mineral Stability and Polymorphs

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    Of the thousands of known minerals, a relative few are very common. A key reason is that many minerals are only stable under specific conditions. In 1878, J. Willard Gibbs defined a form of energy that determines compound stability. We now call it the Gibbs free energy and indicate it by the variable G. Notions involving Gibbs free energy form the basis for the field of thermodynamics. As pointed out by Gibbs, natural chemical systems are most stable when energy is minimized. So, minerals and mineral assemblages with low Gibbs energy are more stable than those with high energy. Consequently, unstable minerals break down to form different minerals, with lower Gibbs free energy, over time. Thus, minerals with relatively low Gibbs energies are more common than others.

    Elements can combine in many ways to form crystals, but as atoms bond, they naturally tend to arrange themselves in the way that minimizes Gibbs free energy. For example, mineralogists have identified more than half a dozen naturally occurring polymorphs of silica (SiO2). Polymorphs are minerals that have identical compositions but different arrangements of atoms and bonds. The polymorph with the lowest chemical energy is the stable form of SiO2. Different polymorphs have the lowest energy under different pressure-temperature conditions. At room temperature and pressure α-quartz has the lowest energy and is stable; the other polymorphs are thermodynamically unstable.

    Under normal Earth surface conditions, because α-quartz is stable, we would expect it to form to the exclusion of the others. This is generally the case, but there are exceptions. For kinetic and other factors, natural systems may not always reach lowest (stable) energy conditions. So, especially at low temperatures, we sometimes find examples of metastable SiO2 polymorphs – these are minerals that should, in principle react to become α-quartz if we waited long enough. At elevated temperatures and pressures, metastable α-quartz cannot persist. It changes into other SiO2 minerals (such as cristobalite, coesite, tridymite or stishovite) that have lower chemical energy (Box 4-2).

    4.25 The difference between α-quartz and β-quartz

    A change from one polymorph to another can be a reconstructive transformation involving major rearrangement of atoms and bonds. Alternatively, it may be a displasive transformation that involves bonds stretching or shrinking (not breaking) and angles between bonds changing, as one mineral turns into another. An example of a displasive transformation is the polymorph change when high-temperature SiO2, β-quartz, turns into α-quartz with cooling. It is very subtle, but, as seen in Figure 4.25, the difference between the two polymorphs is whether the structure contains perfectly hexagonal symmetry and openings. β-quartz does (drawing on the right) and α-quartz does not (drawing on the left). The video below shows the structure of α-quartz. Look closely and you will see that it begins with a view down the channel similar to the photo on the left above in Figure 4.25.

    A third kind of transformations is an order-disorder transformation. These involve atoms ordering and arranging in slightly different ways and are gradual changes that occur over a range of pressures or temperatures.

    Transformations of any kind may occur quickly or may be very slow. Reconstructive transformations are generally sluggish, and may not occur even if they should. For example, all diamonds should turn into graphite (a reconstructive transformation) at Earth’s surface, but they do not. In contrast, displasive transformations, such as α-quartz turning into β-quartz are instantaneous and reversible. Heat a quartz crystal to just over 573 °C and it changes into β-quartz . Cool the same crystal and it will change back into α-quartz. Order-disorder transformations, the third kind of transformation, occur at different rates depending on how fast temperature, and sometimes pressure, change. If the rate of change is fast, no transformation may occur.

    Silica Polymorphs

    4.26 Small crystal fragments of coesite

    Many known minerals have composition SiO2. Some of the most important include quartz, tridymite, cristobalite, coesite, and stishovite. This photo shows grains of coesite; the largest is about 0.4 mm across. The coesite looks a lot like quartz, but has a different internal arrangement of silicon and oxygen atoms.

    For some spectacular scanning electron microscope images of silica polymorphs, see Figure 12.35 in Chapter 12.

    Two of the most important polymorphs, α-quartz and β-quartz, are stable at low pressure. α-quartz is by far the most common of the silica polymorphs because it is stable at room temperature and pressure conditions. Because it is stable at lower temperature than β-quartz, α-quartz is sometimes called low quartz, and β-quartz is sometimes called high quartz. At 1 atm pressure, β-quartz exists only at high temperatures. Upon cooling it will turn into α-quartz at 573°C, so we have no room temperature samples of β-quartz to examine.

    Tridymite and cristobalite are silica polymorphs that, like β-quartz, are found in high-temperature rocks – mostly only in silica-rich volcanic rocks. They are unstable at room temperature and should become α-quartz. However, some samples of these two minerals persist as metastable minerals. For example, the snowflakes that form in volcanic glass (see Figure 4.5) consist of metastable cristobalite crystals.

    Coesite and stishovite are dense silica polymorphs that form at very high pressure. They are unstable at Earth’s surface. Coesite, which forms at pressures above 25 Kbar (equivalent to 75 km depth in Earth), was first found in meteorite impact craters, later in a few eclogite xenoliths from the mantle, and more recently in what are called ultrahigh-pressure (UHP) crustal rocks. Stishovite forms at even greater pressures (and depths) than coesite. We see it today as microscopic grains in meteorite impact craters and in some rare ultra-high pressure rocks. Whether in impact craters, xenoliths, or UHP rocks, both coesite and stishovite often show signs of reacting to become α-quartz, although the reactions do not always go to completion.

    An important corollary to the laws of thermodynamics, the phase rule, says that the number of stable compounds that can coexist in any chemical system must be small. Thus, not only are stable minerals predictable, they are limited to a small number. For a given rock, the stable minerals may not be the same under all conditions. If a rock is metamorphosed due to pressure or temperature changes, minerals may react to produce new minerals with lower Gibbs free energy. When they stop reacting, they have reached equilibrium. If the Gibbs free energy is minimized, the system is at stable equilibrium.

    4.27 Black crystals of magnetite from Bolivia

    Consider a chemical system containing Fe-metal and O2. These elements can exist in their pure forms (as metallic iron and gaseous oxygen), but when mixed, they tend to react to produce magnetite (Fe3O4), or hematite (Fe2O3), perhaps creating minerals like the ones seen in the two photos here (Figures 4.27 and 4.28). Both minerals have lower Gibbs free energies than mixtures of Fe and O2. This same principle applies to more complicated systems involving many elements, for example a rock. For any given composition rock, one (stable) mineral assemblage has the lowest energy. If the rock reaches stable equilibrium, the stable assemblage will prevail.

    4.28 Hematite from the Czech Republic

    Although the laws of thermodynamics tell us what the most stable mineral(s) should be, they do not tell us how long it will take to reach stable equilibrium. We all know from experience with cars, for example, that it may take a while for the iron in steel to rust, even though (unrusted) iron is unstable at Earth’s surface. The same is true for reactions involving minerals. Some low-temperature systems never reach stable equilibrium, and remain in an intermediate stage called metastable equilibrium when reactions cease. For example, countless numbers of diamonds exist at Earth’s surface, although graphite is a more stable form of carbon. Unless diamonds are heated, they remain metastable and do not change into graphite, no matter how long we wait. In contrast with low-temperature systems such as those at Earth’s surface, most higher-temperature natural systems approach stable equilibrium given enough time.

    This page titled 4.3: Mineral Stability and Polymorphs is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform.