CompuServe Messages

Hi, Kamron– I got lucky and found the May, 1992 issue of Rock & Gem magazine, which has a discussion of geode formation (it refers in turn to _Agates_, by Dr. H. G. MacPherson, and _The Origins of Agates, Thundereggs and Other Nodular Structures_, by Ben Schaub). There is some difference of opinion as to whether the blobs of silica gel form with the igneous rock, or whether they arrive later to fill cavities as I described. Because it is a gel rather than a liquid, because it generally cools starting with outside, and because it typically fills the cavity, it deposits quartz (chalcedony) on all surrounding surfaces with little regard for gravity. As it continues to cool, layers of quartz are deposited with slightly different chemistries, thus the banded appearance. The layers do not form fully solidified, but still somewhat plastic. If the temperature/chemistry changes to favor rock crystal quartz forming instead of chalcedony, heat may be released during this crystallization and force the plastic chalcedony layers to curve toward weak points in the containment, forming escape tubes (to allow a pressure release or an expansion of the containment). Stepping beyond the article… When crystals form directly on a solid surface, gravity is more or less irrelevant (gravity dictates the surface of the liquid relative to any accompanying gas or other liquid phase, thus some rock surfaces can be blocked off – but this does not appear to apply in most geodes). Gravity governs crystal formation only when the crystals form in a liquid and then fall thru that liquid. I figure that sometimes the liquid/solid ratio of the gel may be such as to leave it more or less a liquid in the late stages, allowing crystals to form in the liquid and fall to the bottom, forming the flat oriented layers one sometimes finds inside a geode. This may need a sudden temperature shift. Eventually dissolved solids may be exhausted, and/or the gel/liquid escapes through one of these escapement tubes, leaving the cavity often found in the geode (indeed, many call it a "nodule" rather than a "geode" when there is no cavity). –Doug