What are funnel crystals

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Funnel crystals from space

Model of a “funnel cube”, next to it in the small picture an analogue, hollow rock salt crystal (4 mm KL) grown on the ISS. Graphics / Photo: Pettit & Fontana 2019 / npjmgrav

Experiments carried out by NASA astronaut Don Pettit on board the International Space Station (ISS) in close collaboration with Swiss salt specialist Pietro Fontana [This e-mail address is in front of spambots gave an interesting insight into the crystal growth of common salt funnel cubes protected! To display JavaScript must be turned on!]. The elaborate investigations focused on the formation of skeletal common salt crystals, the so-called “funnel cubes”, which arise through diffusion-controlled growth when the mother solution is slightly oversaturated (cf. Lapis 10/2016). In laboratory tests on earth, these halite crystals grow very quickly, but under conditions that can only be sustained for seconds to minutes. Therefore, such crystals are only about ¼ mm in size. In contrast, halite funnel crystals grew significantly more slowly on the ISS under the conditions of extensive - but not more complete - weightlessness, but for days to weeks. This resulted in much larger crystals with an edge length of 2-8 mm. Nature allows even more time: Under terrestrial conditions and during geological periods of time - thousands to hundreds of thousands of years - salt funnel crystals grow "floating" in salt-saturated clay sludge, obviously under conditions that are similar to reduced weightlessness. Edge-emphasized "hopper crystals" even reach sizes of up to 20 cm! Well-known sites for such "giants" are the Dead Sea and the Bistol Dry Lake in California, but also Corocoro in the Bolivian highlands, where, in addition to the classic copper pseudomorphs after aragonite, calcite pseudomorphs after halite funnel crystals also occur very rarely (extraLapis No. 43, pp. 76-81).
Stefan White

Extreme edge growth in nature: Former salt crystal (3 cm), pseudomorphically replaced by calcite and covered with limonite. Corocoro, Bolivia. Anne Cook Collection, Photo: Jesse La Plante