I found a small number of agates at Birdlings Flat and was given a number more by people I met on the beach. I have now tumbled these for one week in 100-mesh silicon carbide grit (see the bottom of this Post for an explanation of what agate is). The interesting results are as follows:
The Gitche Gumee Museum is a rock and mineral museum in in Michigan. Its website has the following explanation about agate and its formation:
“Agate is a form of chalcedony, which is silicon dioxide in the form of microscopic fibrous quartz crystals. Agates naturally develop when an empty pocket inside a host rock, usually volcanic lava, fills in molecule-by-molecule, layer-by-layer, as these microcrystals form concentric bands or other patterns. The colors and arrangement of the microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process. The empty cavities and seams filled with fluids rich in dissolved and suspended quartz molecules (silica), as well as other mineral impurities. When the silica concentration became supersaturated, it developed a gelatin-like consistency either throughout the pocket or in a layer that served as the active crystallization front. Over time, the silica molecules began to form miniature fibrous microcrystals that attached to the sides of the cavity or seam. During the filling-in process other mineral impurities collected at the inside of the chalcedony silica band, forming intervening and often contrasting bands.”
Wikipedia puts it like this:
“Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass, which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Agate has also been known to fill veins or cracks in volcanic or altered rock underlain by granitic intrusive masses. Such agates, when cut transversely, exhibit a succession of parallel lines…giving a banded appearance to the section. In the formation of an ordinary agate, it is probable that waters containing silica in solution – derived, perhaps, from the decomposition of some of the silicates in the lava itself – percolated through the rock and deposited a siliceous coating on the interior of the vesicles. Variations in the character of the solution or in the conditions of deposition may cause a corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of crystalline quartz.”
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