Skip to Main Content
Skip Nav Destination

Much of the Earth’s dynamics is related to mineral reactions in the solid-state. Classically, this is referred to as metamorphic crystallization (Kretz, 1994). Based on the chemical compositions of the phases involved in a metamorphic mineral reaction, two basic reaction types may be distinguished. Reactions that involve only structural re-arrangements, while the compositions of the reactant and product phases are identical, are referred to as partitionless and ‘polymorphic phase transformations’. If, in contrast, one or more reactant phases are replaced by one or more product phases with different compositions, this implies that chemical components are supplied to or removed from the reaction interfaces separating the reactants from the product phases. In the absence of advective transport via afluidor melt, the necessary chemical mass transport can occur only by diffusion. Accordingly, this reaction type is partitioning and is referred to as ‘diffusive phase transformation’. Some treatments of the kinetics of mineral reactions are based on partitionless polymorphic phase transformations and are reviewed only briefly in this chapter. However, because most metamorphic mineral reactions are partitioning diffusive phase transformations, the following discussion will focus mainly on this reaction type.

In this chapter, three types of reactions that play a key role in metamorphic crystallization are addressed. During prograde metamorphism continuous supply of aqueous fluid by dehydration reactions may facilitate relatively rapid inter-crystalline diffusion so that a state close to chemical equilibrium on the scale of mineral grains and beyond may be attained resulting in ‘porphyroblastic mineral growth’. Interface-reaction controlled and diffusion-controlled growth are two end-member models in the kinetics of porphyroblastic growth and differ in terms of the spatial extent of chemical equilibration and its influence on the distribution and compositional zoning of porphyroblasts. The first section of this chapter may serve as a review of some of the key works in metamorphic petrology addressing the factors that control the abundance and size distribution of porphyroblasts and their chemical zoning patterns.

During retrograde stages of metamorphism or during metamorphic overprint of a previously largely dehydrated rock, crystallization may take place in a relatively ‘dry’ environment, where inter-crystalline diffusion is comparatively sluggish. In such a situation, reaction microstructures such as ‘reaction bands’ or ‘corona structures’ may develop. Typically, both the reactant and product phases are present providing evidence of incomplete reaction and indicating an overall disequilibrium situation. Chemical equilibrium may be restricted to microscopic domains along the phase boundaries or may not be attained at all. Nevertheless, important rate and time information may be obtained from the analysis of such reaction microstructures, if the processes underlying their formation are known and their rates are calibrated. The formation mechanisms of reaction bands and corona microstructures are discussed in the second section of this chapter.

Finally, the mechanisms underlying symplectite formation, another phenomenon that is typically associated with metamorphic overprint of magmatic or metamorphic rocks, will be addressed in the third section of this chapter. Symplectites are spatially highly organized, fine-grained intergrowths of two or more different phases, replacing a more coarser-grained precursor phase at a sharp reaction interface. Symplectite microstructures are characterized by a specific length scale of phase alternation, by specific lamellar or rod-shaped microstructure and compositional patterns. In this chapter different avenues for extracting petrogenetic information from symplectite microstructures are discussed.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Close Modal

or Create an Account

Close Modal
Close Modal