Moonmilk (sometimes called mondmilch, also known as montmilch or as cave milk) is a white, creamy substance found inside caves. It is a precipitate from limestone comprising aggregates of fine crystals of varying composition usually made of carbonates such as calcite, aragonite, hydromagnesite, and/or monohydrocalcite.
There are several hypotheses concerning the origin of moonmilk. One of these explains moonmilk to be the result of bacterial action rather than from chemical reactions. According to this particular hypothesis, moonmilk is thought to have been created by the bacterium Macromonas bipunctata. However, no microbiological studies have been carried out so far. Moonmilk was originally explained (by Conrad Gessner, 1555) to be created by "moon rays".
It is possible that moonmilk is formed by water that dissolves and softens the of karst caves consisting of carbonates, and carries dissolved nutrients that can be used by microbes, such as Actinomycetes. As the microbial colonies grow, they trap and accumulate chemically-precipitated crystals in the organic matter-rich matrix formed that way. Perhaps these heterotrophic microbes, which produce CO2 as a waste product of respiration and possibly organic acids, help to dissolve the carbonate.
The discovery of a 150-meter-long moonmilk river in a cavern in Hernio mountain, in the Basque Country of Guipúzcoa province, Spain was announced in March 2004 by a team of speleologists. Its exact location of the cavern is kept secret, so as to avoid vandalism. This river is formed by a thick milk-like fluid containing calcite, quartz and brushite in suspension.
In the middle of 16th century moonmilk was used as a medicine according to Gessner, and continued to be used as such until the 19th century. It is said to have cured acidosis and probably cardialgia by neutralizing an overdose of acid. It had no adverse health effects.
- George W. Moore and Nicholas Sullivan. Speleology: Caves and the Cave Environment, rev. 3rd ed. Dayton, Ohio: Cave Books, 1997. ISBN 0-939748-46-0 (hardcover), ISBN 0-939748-45-2 (paperback).
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- Moonmilk and Cave-dwelling Microbes
- Micromonas bipunctata
- The Virtual Cave: Moonmilk
- Novedades Rio Subterráneo de Leche de Luna (Spanish)
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