Also: Equi-temperature metamorphism, ET, rounds, rounding, sintering, bonding, etc
Crystal growth or metamorphism which occurs at a slow (near-equilibrium) rate in the absence of a large temperature gradient or other strong non-equilibrium factors. Grains become rounded and bonding between grains (sintering) increases.
In the advanced stages the crystals are well-rounded, bonded (sintered), and all very close in size. [Colloqially "Advanced ET"]
This metamorphic process has always been believed to be driven by radius dependent vapor pressure gradients, but this is currently under investigation and is being re-thought by scientists.
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