The shell of a thunderegg.
Nucleation point and spherule.
Well, it is written on the “forming” page, but short: a thunderegg is formed around a nucleation point, which can be a in the lava embedded phenocryst, gasbulb or a xenolith that was picked up by the lava flow. In a cut thunderegg the nucleation point is not always recognizable/visible. In many cases the point is tiny and for this reason it often is missed while cutting the thunderegg. The spherule is the part of a spherulite and lithophyse which surrounds the nucleation point. A thunderegg examined, the spherule often stands out by deviating structure, hardness and/or color, but is not always obvious to spot. The spherule mostly is round, but can also have a composed, concentric shape. It varies in size.
Colburn has been pointing out that growth of cristobalite crystals can result to the forming of spherulites and thundereggs. The radial patterns are visible in lithophyses from several sites. In some cases the patterns become yet visible after a long period of weathering. More frequent the radial patterns are found on thunderegg cores. When a thundereggs shell falls apart because of weathering, a rooftile like print can be found left at the remaining thundereggcore, the former filling of the cavity.
Sometimes the growth of a thunderegg can be interupted and several stages of growth can be observed, as in growth in annual rings in trees. In line of his conclusions, Colburn named them Fronts of Spherulitic Crystallization (FOSC). The fronts are the result of temporary interrupted radial growth of the cristobalite crystals. Occasionally there is change in composition, at which it seems other minerals are embedded between the stages of radial growth. Possibly the interruptions are caused by the change of water supply and/or change in occuring temperature.
At times socalled flow patterns can be perceived in a thundereggs shell. These pattens are the remains of the formerly flowing lava. Flow (banding) patterns are caused by a slight difference in flow speed of the several bands. Some bands have a slightly different composition and therefor do solidify at a higher temperature than other bands. Bands can have a differentiated structure and/or color.
In felsic lava deposits phenocrysts, i.e. Quarz, Feldspar, Oligoklas and Biotote crystals, may occur. They are yet crystallized in the flowing magma, resulting they are also present in spherulites and lithophysae. Lava deposits rich in phenocrysts, the term porphyric is used. When phenocrists are absent, rock is termed aphanic. Generally phenocrists are visible without magnification, but seldom larger then 5 mm.
The presence of flow banding and phenocrysts in thundereggs aswell the hostrock, show that the proces of radial crystallization of cristobalite took place after the outflow but before the solidification of the lava. Exsisting flow patterns were captured in spherulites and thundereggs. Other evidence of the more or less viscous state of the lava during the forming of thundereggs, are flow stretched and twisted thundereggs.
Many thundereggs do have a rounded shape, the size can be rather various. Some sites have eggs with minor difference in size, although usually there is variety. Smaller eggs with sizes up to about 1 cm. can occur in conglomerates, packed densly in the hostrock. Rarely thundereggs have a diameter of more than 1 meter, generally the size is 4 – 12 cm.
The exterior of a less weathered aphanic thunderegg can be very smooth, an irregular shape of the outside is uncommon. Porphyric lithophysae have a more rough look. A wrathy exterior is caused by spherulites because small spherulites got attached at the thunderegg. In some cases it looks like there has been a renewed spherulitic growth, whereby small spherulites have been growing onto the main thunderegg. A special (and very sought for) form of the renewed growth is the socalled Turkeytail thunderegg, at which several rowes of spherulites are formed around the main egg.
Flat, lens shaped thundereggs are found at few sites, probably the form is the result of the weight of a top layered deposits. The rarely seen tube shaped eggs can, according to Peter Wörner, develop in two ways. Flow-stretched thundereggs (Axioliths, Torpedo’s) have been influenced during the forming by flowing lava. In such egg only one nucleation point that often is stretched, and one cavity is present. Chained thundereggs do have more than one nucleationpoint. The long shaped eggs exsist of several lithophysae of which the nucleation points are lieing in line, close to eachother. The eggs have been fused during the growth, cavities can be connected but have rather a complex shape.
A thundereggs shell color sometimes gives, in combination with the eggs filling, an attractive picture. Bright colors are rare, The ‘Killer Green’ in the Harvey Gap thundereggs, the burning red in eggs from Lead Pipe Springs, the colorful Mariposa eggs are exceptions. The shell can be colored by mineralizations containing Manganese (black, dark grey), and iron as Marcasite (grey), Limonite (yellow, orange, brown and red). A green color can be a result of the presence of Seladonite. Sometimes coloration can be very local, along cracks or around inclusions.
Due to weathering a change in color can derive. A thundereggs shell has a tendency to bleach after a very long period of weathering. When a shell is rich in iron minerals, the exterior can be red colored due to oxidation.