| | Microstructures of prograde mineral growth in the Bushveld aureole |
 | | The sequence inferred from microstructures shows considerable overlap of the period of growth of porphyroblasts of staurolite, cordierite, biotite and andalusite, and the persistence over a similar interval of the reactant porphyroblastic phase chloritoid. |
 | | This is inconsistent with calculations of equilibrium phase relations, and implies that disequilibrium processes controlled the prograde reaction sequence, despite the slow heating rates involved (1°C per 10,000 years). |
 | | The early appearance of cordierite by a metastable reaction and its subsequent disappearance indicates that delayed nucleation of porphyroblastic phases, rather than simply sluggish reaction, is required to account for the sequence of growth. |
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