ABSTRACT
Crystallisation of metamorphic rocks in response to changing P–T conditions requires crystallisation of minerals by nucleation and growth. Homogeneous nucleation, involving spherical nuclei with uniform surface energy, randomly distributed throughout the host in which they develop, may be relevant in chemistry and metallurgy, but is an inappropriate description of nucleation in heterogeneous polycrystalline metamorphic rocks. Heterogeneous nucleation describes non-random nucleation on some pre-existing substrate, such as new crystals preferentially nucleating at preexisting grain boundaries. Even the purest monomineralic quartzite or marble has heterogeneities such as microfractures or small detrital grains. Because of this there will always be some places where new crystals are more likely to nucleate, and other areas where nucleation is less likely. In view of this, heterogeneous nucleation is the most relevant way to consider nucleation in metamorphic rocks. Bulk rock chemistry also plays a key role in influencing the number and size of porphyroblasts that develop. An example from the aureole of the late Caledonian Corvock granite (Co. Mayo, Ireland) is shown in Fig. 5.1. In this case, cordierite is preferentially developed as larger porphyroblasts in pelitic compared to semipelitic horizons. Metasediments in the contact aureole of the Corvock Granite, Mayo, Ireland, showing widespread cordierite development. Due to bulk rock chemical controls on nucleation and growth, porphyroblasts have preferentially developed to a larger size in the pelitic layers compared to the semi-pelitic layers. The lens cap is 45 mm in diameter. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315831626/67ff2ae0-2ee0-4d05-9e97-aa4df451c794/content/fig5_1_B.jpg" xmlns:xlink="https://www.w3.org/1999/xlink"/>
