Recrystallization

Once a mineral has crystallised, that is not necessarily its final state. Provided temperatures are high enough to allow atoms and ions to migrate, a mineral will continue to change in ways that lower its total energy. One of the most important of these post-crystallisation processes is recrystallisation - changes to the size and shape of already-formed mineral grains, together with the collection of processes that remove defects from crystal structures. ^[1] Recrystallisation is one of the dominant processes shaping the texture of metamorphic rocks, and its products are directly readable in thin section as grain size, grain shape, and boundary geometry.

The Driving Force: Surface Energy

The surface of any crystal is a higher-energy configuration than its interior. The reason is simple: atoms at the surface have incomplete sets of chemical bonds. Interior atoms are fully bonded on all sides, occupying a low-energy environment. Surface atoms are bonded to neighbours within the crystal but face open space (or another mineral) on the other side, leaving some bonds unsatisfied or distorted. This excess energy at the surface is called surface energy, and it is directly proportional to the total surface area. ^[1]

Because the system always seeks lower total energy, minerals will recrystallise in ways that reduce total surface area. Two mechanisms accomplish this: smoothing irregular grain boundaries, and allowing some grains to grow at the expense of neighbouring smaller grains. ^[1]

Grain Boundary Smoothing

Where two mineral grains meet along an irregular boundary, the total surface area of that boundary is greater than it would be if the boundary were perfectly flat. Every bulge and indentation on an irregular surface adds to the total area - and therefore to the total surface energy. Given sufficient temperature to allow ion migration, atoms diffuse to fill in the low spots and remove the high spots, gradually flattening the grain boundary until it is smooth. The required temperatures are found both in magmatic environments and during metamorphism; quartz and feldspar allowed to recrystallise at high temperatures typically develop a granular texture bounded by smooth surfaces as a result. ^[1]

Grain Coarsening

The second mechanism is grain coarsening: smaller grains dissolve and the atoms they release are incorporated into larger neighbouring grains. This process reduces the number of grains in the rock, increases their average size, and - crucially - decreases total surface area. A simple numerical example makes the magnitude of the effect clear. A collection of 1,000 grains each with a radius of 0.1 mm has a total surface area of 126 mm². If all the material from those 1,000 grains were combined into a single grain with a radius of 1 mm, the total surface area would drop to only 12.6 mm² - a tenfold reduction in surface area from a tenfold increase in grain size. ^[1]

The practical consequence of this driving force is that grain size in metamorphic rocks increases systematically with increasing temperature. The clearest documented example is provided by marble in contact metamorphic aureoles. Marble immediately adjacent to a hot intrusive contact was subjected to the highest temperatures, and it shows the coarsest calcite grains. Marble farther from the contact experienced progressively lower temperatures and shows progressively finer grain size. This spatial pattern is not coincidence - it is the direct record of the temperature gradient around the intrusion, preserved in grain size. ^[1]

Defect Removal and Annealing

Grain boundaries and grain surfaces are not the only high-energy features that recrystallisation addresses. Dislocations - both edge and screw varieties - represent higher-energy configurations than a defect-free lattice, because they involve local distortion of atomic positions and unsatisfied or strained bonds. Lattice distortion produced by tectonic deformation similarly represents excess energy stored in the crystal. Given sufficient temperature, atoms will migrate within the crystal structure to remove these defects, restoring the lattice toward its ideal, lowest-energy geometry. This defect-removal process is a form of annealing. ^[1]

This is why strongly deformed minerals in a rock that is subsequently heated - for example, by burial or by a nearby intrusion - often show evidence of annealing: straight, clean grain boundaries replacing the irregular, strained boundaries of the deformed precursor, and optically uniform grain interiors replacing the patchy undulose extinction of strained grains. Recrystallisation thus erases the record of deformation while simultaneously recording the temperature of the thermal overprint.

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