If I would ask you what you see in the picture above, some of you might see a leaf, a fossil leaf that fell from its tree 50 millions years ago. You might recognize a leaf blade in the central part, and around it the symmetric veins that branch toward the relatively well defined margin. Let’s compare it to a typical leaf morphology: Analogies are quite evident! Other people might relate such a symmetrical structure to a lake where the central line corresponds to the water surface. The upper part is reflecting on the lake surface which gives the effect of symmetry.
However, looking carefully at figure 1, it can be noticed that the intriguing structure is not as symmetric as it seems and that it is relatively irregular. Figure 2 shows a fossil leaf of fern from the fossils collection at the Natuurhistorisch museum, Maastricht. The structure in figure 1 is clearly different from this fossil leaf. A striking feature is the relief, which might be challenging to recognize in figure 2. The “curious structure” in figure 1, indeed, is not a fossil leaf. Two different colours can be identified, from red-brown to white. The brownish material is in addition characterized by a lower relief compared to the whitish layers.
But then, what is this structure?
It is an olivine crystal, the most abundant constituent of the Earth’s upper mantle. Nonetheless, this is not its most usual form. This particular crystal was formed at disequilibrium conditions. Hmm, “disequilibrium”.. Let’s see what we mean with this word… Certain conditions such as temperature, pressure, chemical composition etc. control the crystal growth. Often, when these conditions (i.e. pressure and temperature) are favourable for the development of a well-formed geometry, olivine crystallizes from melt as rounded and rectangular crystals (figure 3). A pointed top and easily recognizable faces (the relatively flat surfaces by which a crystal is bound) are formed, like for the blue crystals in figure 3. However, sometimes temperature conditions may vary during crystallization. This then changes the velocity at which crystals grow. In order to have an idea of crystallization velocities (measured as temperature (in °C) per time (in hours)), a well-formed crystal can occur when the temperature decrease ranges between 2 and 50 °C/h.
Conversely, melt crystallizing in a magma chamber can reach crystallization rates of 2000°C/h, when the temperature of the melt is much higher than the temperature of the rock surrounding it. These conditions are defined as disequilibrium conditions, related also to the significant difference in chemical composition between melt and rock. Time is too short for the melt to equilibrate with it. The rapid growth of olivine crystals in such systems results in the missing development of certain structural component such as faces of the crystal. This is well illustrated in figure 4: olivine crystals show concentric empty terraces where crystal faces should have formed. At crystallization velocities higher than 100-200°C/h olivine crystals grow as elongated and branching structures as the one in figure 1. These olivine crystal forms are called harrisitic or skeletal and can reach large dimension, with crystals as long as one meter, as described in the Rum Igneous Centre (Scotland).
Back to the “curious structure” (figure 1): the brown material is olivine that is now altered since its air and rain exposure, whereas white is plagioclase. We learned here that we should further investigate any structure we observe for the first time and should not stop at our first impression. This is one of the reasons why our research as geologists, and more in general scientists, is so important.
More informations on mineral formations can be found here.
I leave you with this last image (figure 5): is this an olivine, or is this a leaf? Or anything else?
Please leave a comment if you wish!