Moonmilk

Moonmilk (sometimes called mondmilch, also known as montmilch or as cave milk) is a white, creamy substance found inside limestone 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 as created by "moon rays".

It is possible that moonmilk is formed by water that dissolves and softens the karst of 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.

Being soft, moonmilk was frequently the medium for finger fluting, a form of prehistoric art.

The world's largest formation of brushite moonmilk is found in the Big Room of Kartchner Caverns State Park in southern Arizona.

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