Circles of Mystery - Agates, Geodes and Spherulites
Agates develop as secondary deposits in hollow cavities, called
Most of the world's agates developed in ancient volcanic lava.
Silica rich fluids fill a cavity and as the liqid cools down the silica
In an agate the silica deposits over the entire inner surface of the
The fluid can cool quickly, drain, then heat up and fill the cavity again
Different colours are caused by changes in the fluid chemistry over time.
Geodes also form in vesicles (hollow cavities ), mostly in volcanic
However when the silica rich fluids cool , they find points of nucleation
(sites where individual crystals can grow.
The crystals grow inwards from the walls arth right angles.
The fluid maintains a steady temperature over a period of time.
The longer the time the bigger the crystals can get.
Spherulitic Dolerite is formed by igneous processes.
Essentially a large mass of diorite rock begins to cool and form acicular
(needle -like crystals of plagioclase feldspar.
But then the cooling regime changes and the mass of diorite rock undergoes
large supercooling (like glass).
The acicular plagioclase crystals aggregate (collect together) and form
The largest crystalsform on the insdie of the sphere and as the rock mass
successive layers of smaller and smaller crystals adhere and form
concentric ring-like sturctures.