Geode (Earth Sciences)

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A geode ( old Greek γεώδης geōdēs , earlier also Gäode , is a not uniformly used term from geology and mineralogy , with which, depending on the source location, a round cavity, bounded by a uniform rock outer layer, and / or by different, Geological processes resulting filling with mineral or fossil substance is called. If a residual cavity remains after a filling with crystals, this is also referred to as a gland and, when it is completely filled, as an almond .

In Petrology one understands a usually under the term Geode concentrically constructed concretion .

Emergence

Formed in volcanic rock

Within a lava flow of between 800 and 1200 ° C , the volatile components are released from the rock melt and initially form small gas bubbles . While these bubbles are still mobile for some time in the hotter core of the lava and can combine to form more or less large cavities as they migrate towards the surface, they are fixed relatively quickly in the faster cooling edge areas and remain small with mostly rounded or lens-like shape. The diameter of the bubbles and subsequent geodes can be between a few centimeters and several meters. One of the largest known geodes, which was discovered at the end of 1999 in the Mina Quien valley Pensara ( Mina Rica ) near Pulpí in the Spanish province of Almería , is lined with large, perfect and transparent gypsum crystals ( Marienglas ), has an oval diameter of 1.8 × 1.7 meters, a length of 8 meters (internal dimensions) and can accommodate up to 10 adults. A research team led by Juan Manuel García-Ruiz was able to narrow the age of the so-called "Geode of Pulpí" to about 60,000 to 2 million years, the lower age limit being determined on the basis of a few carbonate crusts that have deposited on some gypsum crystals.

Since lava flows can flow for a relatively long time during the cooling process, the gas bubbles are drawn out in the direction of flow. In doing so, they take on a drop - to almond - like shape with a thinner tip pointing upstream.

With further cooling to around 400 ° C, depending on the composition of the volcanic gas , it condenses to form aggressive, partially aqueous ( hydrothermal ) solutions that attack and decompose the surrounding rock. In this way, on the one hand, tiny connecting channels are created between the bubbles and in the rock, and on the other hand, the first mineral fillings in the geodes from the highly separating components of the solution. Further subsequent hydrothermal processes, fed by the continued volcanic and tectonic processes in the earth's interior, continue the work of cavity filling or weathering, until finally after several tens to hundreds of millions of years drusen with partial mineral filling and remaining cavity in the center or completely filled almonds have formed .

The edge of the cooled lava flow, consisting of many smaller and larger solidified gas bubbles (geodes), is also known as the almond stone . In cross-section, the lava flow consists of a gas-free, compact core that is covered on all sides with a layer of almond stone.

  • Geode in the bedrock, partially filled with amethyst crystals (druse)

  • Cut open geode with quartz crystals

  • Giant detached geode with amethyst lining, exhibited in the National Museum of Scotland

Sedimentary geodes

Geodes can arise in the sedimentary formation area through precipitation from the pore water and from flowing water around a sedimentation nucleus , which usually consists of organic residues. The precipitation is triggered by differences in the pH value around the crystallization nucleus, which is usually fossilized in the process. The precipitated material usually differs in substance from the mineral in the immediate vicinity. The deposition is usually concentric and reflects the changing deposition conditions during rock formation. Clay iron odes can often be observed in the sedimentary area , but carbonate minerals , pyrite or marcasite and various quartz varieties are also involved in the structure of the geodes. Geodes, like septaries, belong genetically to the concretions.

Synonymous and misleading use of words

Agate almond with quartz gland

Geode and Druse are often used synonymously, especially when describing different types of quartz that occur in nature, such as amethysts and agates . For example, the term "agate geode" describes a rounded rock body, the bark of which consists of agate and which has quartz, amethyst or other free-standing minerals in a remaining cavity. Alternatively, the term almond with druse is used for this mixed form.

Some authors also use the term for cavity fillings created by secretion (see also in Mandelstein ), other authors restrict the meaning less strongly and use the term for concretions in general.

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

literature

  • Ursel Laarmann, Michael Landmesser, Maximilian Glas, Rupert Hochleitner, Rudolf Dröschel, Peter Jeckel: Agate: The precious stone from which Idar-Oberstein was created: history, origin, finds . In: Christian Weise (ed.): ExtraLapis . tape 19 . Christian Weise Verlag, 2000, ISBN 3-921656-54-0 , ISSN  0945-8492 , p. 26 to 28 .
  • Pierer's Universal-Lexikon, Volume 5. Altenburg 1858, p. 352. ( online copy )
  • Kitty L. Milliken: Geodes . In Gerard V. Middleton: Encyclopedia of Sediments and Sedimentary Rocks . Springer 2003, ISBN 978-1-4020-0872-6 , pp. 306–308 ( limited online copy in the Google Book Search USA )
  • Walter David Keller: The Common Rocks and Minerals of Missouri . University of Missouri Press 1961, ISBN 978-0-8262-0585-8 , p. 67 ( limited preview in Google Book Search - USA )

Web links

Commons : Geode (Druse)  - collection of images, videos and audio files
Wiktionary: Geode  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Stefan Schorn and others: Mina Quien Valley Pensara (Mina Rica) and "Corta San José". In: mineralienatlas.de. Mineral Atlas , accessed October 18, 2019 .
  2. ↑ Cynthia Reynolds: Messinian Crystals. In: solvitur.de. June 12, 2000, accessed October 16, 2019 .
  3. ^ Thomas Krassmann: Mineral & Exploration - A Giant Gypsum Geode in Almeria, Spain. In: mineral-exploration.de. September 20, 2019, accessed October 16, 2019 .
  4. ↑ Lars Fischer: Climate formed bizarre crystal grotto. In: Spektrum.de. Spectrum of Science , October 16, 2019, accessed October 16, 2019 .
  5. ^ Author collective: Lexicon of Geosciences . tape III . Spektrum Akademischer Verlag, Heidelberg, Berlin 2000, ISBN 3-8274-0422-3 , pp. 152 .
  6. ↑ Agate terms explained in their own words. In: vfmg-weiden.de. Association of Friends of Mineralogy and Geology e. V. (VFMG), June 23, 2004, accessed on October 16, 2019 .
  7. ↑ Murawski, H., Meyer, W. (2004): Geological Dictionary. Spektrum Akademischer Verlag, 11th edition, 262 pp. ISBN 3-8274-1445-8