Geode

A Geode ( altgr. γεώδης geodes, erdartig ' ), formerly also Gäode, is a non- uniform term used in geology and mineralogy, which depending on the source position a rounded cavity bounded by a uniform rock outer layer, and / or by various geological processes resulting filling with mineral or fossil substance is called. Remains at a filling crystals by a residual cavity, this referred to as Druze and when completely filled as almond.

In petrology is meant by the term Geode a concretion usually concentrically constructed.

Formation

Formation in volcanic rocks

Within one between 800 and 1200 ° C hot lava dissolve the volatile components from the molten rock and initially form small gas bubbles. While these bubbles in the hotter core of the lava are still some time mobile and can be connected during their migration towards the surface to be more or less large cavities, they are fixed relatively quickly in the fast cooling border areas and remain small with usually roundish or lenticular shape. The diameter of the bubbles and later Geodes can be between a few centimeters to several meters. One of the largest known geode, which was discovered in Spain in 1999, is lined with large, complete and transparent gypsum crystals ( selenite ), has an oval diameter of 1.8 × 1.7 meters, a length of 8 meters and can accommodate up to 10 adult humans absorb.

Since, however, relatively long lava are flowable during the cooling, the gas bubbles are moved in the flow direction in the length. They take on a drip - up almond -like shape whose thinner tip points upstream.

Upon further cooling to about 400 ° C, depending on the composition of volcanic gas, condenses this to aggressive, partially aqueous ( hydrothermal ) solutions that attack the surrounding rock and decompose. In this way, on the one hand created tiny connecting channels between the bubbles and in the rock, on the other hand first mineral fillings in the geodes from the highly -secreting components of the solution. Other subsequent hydrothermal processes, fed from the continuing volcanic and tectonic processes in the Earth's interior, with its work of cavity filling or weathering continued, until finally, after several tens to hundreds of millions of years drusen with partial mineral filling and residual cavity in the center or completely filled tonsils caused.

The many smaller and larger solidified gas bubbles ( geodes ) existing edge of the cooled lava flow is also called almond stone. In the cross section of the lava thus consists of a gas-free, compact core enveloped on all sides with a layer of amygdaloid.

Sliced ​​Geode with Quartz Crystals

Out Dissolved giant geode with Amethystauskleidung, in the National Museum of Scotland

Sedimentary geodes

Geodes can arise around a mostly consisting of organic radicals Sedimentationskeim in sedimentary formation space by precipitation from the pore water and from rivers. The precipitation is triggered by pH differences to the nucleation, the states is usually fossilize. The precipitated material differs in the ingredient usually from mineral constituents of the immediate area. The attachment is usually concentric and reflects the changing conditions during deposition of the rock formation resists. Frequently in the sedimentary area Toneisensteingeoden are watching, but also carbonate minerals, pyrite or marcasite and various Quarzmodifakationen are involved in the formation of geodes. Geodes are like the septaria genetically to the concretions.

Situated cuts Geode from the Wadi el- Battich

Septaria from Muddy Creek, Utah

Synonymous and misleading use of words

Frequently Geode Druze and are used interchangeably, especially in the description of various naturally occurring quartz varieties such as amethyst and agates. So a rounded rock body, for example, the term " Agate Geode " described whose bark is made ​​of agate and has formed inside quartz, amethyst or other freestanding minerals in a remaining cavity. Alternatively, the term is used with almond Druse for this mixed form.

Some authors use the term also for damages caused by secretion cavity fillings (see also in amygdaloid ), other authors less limit the meaning strong and use the term for concretions in general.

The word geode stands for Earth-like (from Greek geos = earth) and refers to the round Earth-like shape of the stone.