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Contents

A.           Introduction. 8

B.  Occurrence of lithophysae (thundereggs) 8

1.  What is a lithophysa?. 8

2.  Geologic settings of thunderegg development 9

Figures 2.1:  The Priday Beds. 11

Figures 2.2:  Empty thunderegg cavities of the Polka Dot Mine. 12

Figure 2.3: Lava flow sketch. 13

Figures 2.4:  Thundereggs in-situ. 13

Figures 2.5:  Thundereggs with adjacent host rock. 16

Figure 2.6: Rhyodacitic ignimbrite sketch. 17

3.   Development of thundereggs. 17

4.   Size, shape and surface of thundereggs. 18

Figures 4.1:  Big thundereggs (megaspherulites) 19

Figures 4.2:  Elongated thundereggs. 22

Figures 4.3:  Flat thundereggs. 25

Figures 4.4:  The surface of thundereggs. 25

Figures 4.5:  Hemispherulites on the surface of thundereggs. 26

C.  Phenomena of thundereggs shells. 27

5.  Nucleation site (germination) and central spherules (“central buttons”) 27

Figures 5.1.  Phenocrysts as nucleation points. 28

Figures 5.2:  Central spherule with opposing dimple. 29

Figures 5.3:  Central spherules (buttons) of different localities. 31

Figures 5.4:  Central discoloration in thundereggs of St. Egidien. 34

Figures 5.5:  Thunderegg with growth interruption or small spherulite as core for a larger one?. 35

Figures 5.6:  Multiple “central” spherules in the rhyolite shell of thundereggs. 36

Figures 5.7:  Central spherules in thundereggs from Zwickau-Planitz, Saxony, Germany. 38

6.  Xenoliths. 40

7. Radiating structures in spherulites and lithophysae. 45

Figures 7.1:  Small spherulites and lithophysae with radially fibrous crystallization in host rock. 47

Figures 7.2: Small lithophysae without macroscopically detectable radial structure in the host rock. 50

Figures 7.3: Orbicular jasper 52

Figures 7.4: Mushroom Jasper 53

Figures 7.5:  Radiating Patterns in Rhyolite. 54

Figures 7.6: Radiating patterns (plumes) between flow bands. 56

Figures 7.7: Curved radial pattern in rhyolite shells. 58

Figures 7.8: Radial pattern on weathered thunderegg cores. 58

Figures 7.9: Radiating structures on the walls of cavities which remained empty or on chipped off lids. 60

8.  Flow structures in the shell of lithophysae and in the volcanic host rock. 60

Figures 8.1: Flow lamination of the rhyolite shell 63

Figures 8.2: Flow laminations indicating movement and vortex formation. 63

Figures 8.3: Volcanic glass with crystallized flow banding. 64

Figures 8.4: Transition between flow laminations and radial crystallization. 64

Figures 8.5: Glide planes (early crystallized thick flow laminations) 65

Figures 8.6: Difficulties for the radial crystallization to overcome glide planes. 66

Figures 8.7: Continuous flow lamination. 66

Figures 8.8: Flow lamination within adjacent host rock. 69

Figures 8.9: Bending outwards of the flow lamination by the cavity expansion. 70

Figures 8.10: Apparently concentric flow structures caused by cavitation. 71

Figures 8.11: Rupture of the flow banding by the cavity expansion. 72

9.  Observations with phenocrysts in the rhyolite shell of thundereggs. 72

Figures 9.1: Influencing of the flow structures by phenocrysts. 73

Figures 9.2: Nature and size of phenocrysts. 74

Figures 9.3: Disturbance of the cavity expansion by phenocrysts. 75

Figures 9.4: Phenocrysts at the border between rhyolite shell of thundereggs and the host rock. 75

10.  Color of the rhyolite shell 76

Figures 10.1: Color variations of the rhyolite shell of thundereggs. 77

Figures 10.2: Difference between surface and matrix color 80

Figures 10.3: Color changes within the rhyolite shell 81

D.  Thunderegg cavities. 84

11.  Development of the cavity of thundereggs and their pressure ridges. 84

Figure 11.1: Thunderegg core. 86

Figure 11.2: Lithophysal cavity. 86

Figure 11.3: Thunderegg size versus burial depth. 87

Figure 11.4: Thunderegg shapes. 87

Figures 11.5: Pressure ridges on the surface of lithophysae. 88

Figures 11.6: Thin rhyolite coat over prominent pressure ridges. 89

Figures 11.7: Pressure channels and intermediate pressure ridges with inflections. 91

12.  Megaspherulites (Thundereggs without a central cavity) 92

Figures 12: Thundereggs without a central cavity. 93

13.  Biconoids and Triconoids. 93

Figures 13.1: Widely expanded biconoids. 95

Figures 13.2: Narrow biconoid cavities. 96

Figures 13.3: Equatorial pressure ridge of biconoids. 98

Figures 13.4: Biconoids with weathered rhyolite shell or empty cavities. 99

Figures 13.5. Triconoids. 100

14.  Agate stars. 102

Figures 14.1: Development of star-shaped cavities. 103

Figures 14.2: Four-pointed star or “box”-cavities. 105

Figures 14.3: Five-pointed stars. 107

Figures 14.4: Multi-pointed stars. 109

15.  Concentric cavities and onion shell structures. 113

Figures 15.1: Onion skin structures. 115

Figures 15.2: Fronts of spherulitic crystallization. 116

Figures 15.3: Rupture chambers. 118

Figures 15.4: Detached central cores ("egg in an egg ...") 121

Figures 15.5: Onion shells. 123

Figures 15.6: Onion skins in rhyolite shells with many phenocrysts. 125

Figures 15.7: Central onion shells preceding the development of a larger cavity. 127

Figures 15.8: Onion shells by rotational stress?. 128

Figures 15.9: Combination of onion shells (FOSCs), flow lamination and radial crystallization. 129

16.  Combination of a central star with a peripheral ring or crescent ("atoll agate") 130

Figures 16.1: Atoll agate of St. Egidien and Heiterer Blick, Saxony, Germany. 130

Figures 16.2: Atoll-agate from other localities. 132

17.  Irregular cavities, veins and amygdales in silica-rich volcanic rocks. 134

Figures 17.1: Intermediate and transitional forms of the cavity. 135

Figures 17.2: Atypical forms of cavity. 137

Figures 17.3: Amygdales in acidic volcanic rocks?. 139

Figures 17.4: Transitional nodules. 140

18.  Agate emulsion. 140

Figures 18.1: Agate emulsion. 142

Figures 18.2: Mini agates and multi-chambered agate areas. 148

Figures 18.3: Small lithophysae in intrusions. 149

19.  Shearing, ruptures and intrusions. 150

Figures 19.1: Shearing of Thundereggs. 152

Figures 19.2: Destruction before the cavity filling. 154

Figures 19.3: Pressed in segments. 157

Figures 19.4: Cracks and destruction after the cavity filling. 158

Figures 19.5: Intrusions. 160

F.  Mineralization of the cavities. 169

20.  Jasper, jasper-like formations and floor breccias. 169

Figures 20.1: Basal horizontally limited (green) jasper 170

Figures 20.2: Jasper of various colors. 172

Figures 20.3: Ring-shaped structures in jasper and morrisonite jasper 175

Figures 20.4: Jasper with sedimentation structures. 178

Figures 20.5: Non-horizontal surface of jasper 180

Figures 20.6: Formation of a meniscus. 183

Figures 20.7: Two  generations of jasper 184

Figures 20.8: Shrinkage cracks in jasper 185

Figures 20.9:  Not silicified “precursor of jasper”. 187

Figures 20.10: Jasper and opal after a wall-lining agate band. 188

21.  Fortification agate. 190

Figures 21.1: Silica accumulation by diffusion and maturation (Landmesser) 193

Figures 21.2: The concept of flood filling. 194

Figures 21.3: Development of fortification agate bands out of spherulites. 194

Figures 21.4: Fortification agate holospherulites. 196

Figures 21.5: Fortification agates. 198

Figures 21.6: Shadow effect in fortification agate. 203

Figures 21.7: Iris Effect 204

Figures 21.8: Primary green band of phyllosilicates. 205

Figures 21.9: Primary wall-lining band. 206

Figures 21.10: Asymmetric layering of fortification agate. 208

22.  Supply by infiltration channels star rays and ruptures?. 210

Figures 22.1: Infiltration Channels. 213

Figures 22.2: Infiltration channels beginning (or ending) at a primary wall-lining band of chalcedony. 214

Figures 22.3: Open infiltration channels. 215

Figures 22.4: Early quartz crystal layers and infiltration channels. 216

Figures 22.5: Filling of infiltration channels. 219

Figures 22.6: Infiltration Channel in waterlines. 221

Figures 22.7: Iron oxide discs in an infiltration channel 222

Figures 22.8: Cavity filling through cracks?. 222

23.  Flame agate or “hydrothermal” agate. 224

Figures 23.1: Complete cavity filling with flame agate and quartz. 225

Figures 23.2: Flame agate formation before and after other quartz varieties. 227

24.  Waterline agate (Uruguay agate) 235

Figures 24.1: Waterlines from different sites. 237

Figures 24.2: Partial filling by waterlines resulting in a “floor”. 239

Figures 24.3:  Cavities completely filled with waterlines. 241

Figures 24.4: Change between waterlines, fortification agate and other varieties of quartz. 243

Figures 24.5: Waterlines with inconstant width of layers. 248

Figures 24.6: Particles and crystals in waterlines. 248

Figures 24.7:  Discrepancy between horizontal waterlines, the direction of stretching and/or the flow lamination of the thunderegg  250

Figures 24.8: Tilting of waterlines. 252

Figures 24.9: Variable high level positions of waterlines in separate cavity parts. 256

25.  Quartz. 257

Figures 25.1: Quartz crystals as the final deposition. 258

Figures 25.2: Alternating quartz and agate. 264

Figures 25.3: Stalactites of quartz crystals. 266

26.  Opal 267

Figures 26.1: Opal 268

Figures 26.2: Blue Common Opal 273

27.  Other paragenetic minerals in lithophysae. 274

Figures 27.1: Calcite, barite, fluorite, apatite. 274

Figures 27.2: Zeolites. 279

Figures 27.3: Iron minerals. 281

28.  Pseudomorphs. 283

Figures 28.1: Pseudomorphs after calcite and barite. 285

Figures 28.2: Black pseudomorphs after carbonates. 291

Figures 28.2: Black pseudomorphs after carbonates. 291

Figures 28.3: Pseudomorphs after aragonite, fluorite, gypsum, chabazite. 294

Figures 28.4: Needle-like and columnar (sagenitic) pseudomorphs. 296

Figures 28.5: Paraiba-like structures. 304

Figures 28.6: Kinked and broken pseudomorphs. 309

Figures 28.7: Curved pseudomorphs. 309

Figures 28.8: Different fillings of neighboring or connected cavities?. 313

29.  Moss, plumes and dendrites. 315

Figures 29.1: Moss in incompletely filled thunderegg cavities. 316

Figures 29.2: "Eyes" as a cross-section of moss and filaments. 318

Figures 29.3: Moss in thundereggs from various locations. 319

Figures 29.4: Plumes. 326

Figures 29.5: Grainy and fuzzy structures in thunderegg cavities. 329

Figures 29.6: Iron oxides and hydroxides adjacent to the cavity wall 330

Figures 29.7: Mossy opal?. 333

Figures 29.8: Dendrites. 333

Figures 29.9: Wall-lining detachment 335

30.  Colors of the agate. 335

Figures 30.1: Black color 337

Figures 30.2: Red and yellow coloration. 343

Figures 30.3: Liesegang rings in lithophysae. 348

Figures 30.4: Iron oxide discs. 349

Figures 30.5: Discoloration by heating. 351

Figures 30.6: Changes in color after cutting. 351

Figures 30.7: Incipient weathering. 353

31.  Complex cavity filling. 355

Figures 31: Specimens with combined, complex fillings. 355

32.  Supplementary thunderegg images from different localities worldwide. 362

G.  Summary. 400

H.  Glossary. 401

 

 

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