Lierbachtal, Black Forest, Germany. 178 gram, 102x58 mm. Cut half of a stringer of thundereggs, pictured wet. Yesterday, on his FB page, Rob van Leeuwen theorized somewhat on the structure of thunderegg shells. T-eggs are considered to be growing structures. Robert Colburn thought the shells are made up of a quartz variety called cristobalite, growing outwards in feathery, radiating patterns (which can be seen in the shells of e.g. Buchanan Ranch t-eggs). He also thought this growth happens in phases, so called fronts of spherulitic crystallization, defined as the concentric banding that forms during cristobalite crystallization as concentric layers of radial crystals [The Formation of Thundereggs, ed. Kasper, p. 260]. The problem is, that this phenomenon, although clearly visible in a lot of spherulites, is much less obvious in thundereggs. Indeed, there are deposits where you find t-eggs that appear to have halted growing at one point, and started it up again later on, like Laguna t-eggs. Or (clusters of) t-eggs that seem to have been the nucleus for a larger lithophysal structure (Highway 46, California; mushroom jasper, Arizona). But these could be exceptions, rather than proofing a rule. Another shell feature is the so called flow-banding, a pattern of lining caused by movement in the flow. Dugway geodes are often good examples. This lining can also be seen on the outside of the t-egg. In theory the area around the nucleus (e.g. buttons in Priday flow t-eggs), and where fronts of spherulitic crystallization and/or flow bands connect are weaker points in the shell structure, that can rupture to form one or more cavities. And then I look at a specimen like this Lierbach t-egg cluster. Im afraid it gets me more questions than answers...
Posted on: Mon, 22 Dec 2014 17:30:36 +0000