Formula: Ca(PO3OH).2H2O
Hydrated acid phosphate
Specific gravity: 2.30 to 2.33
Hardness: 2½
Streak: White
Colour: Colourless, light yellow; colourless in transmitted light
Solubility: Readily soluble in hydrochloric acid

Cave deposits

Brushite is one of the most common cave minerals, in guano deposits, and in phosphorite, formed at low pH (acid) by reaction of phosphate-rich solutions with calcite and clay (HOM). It also occurs as incrustations on ancient bones and crypts. It may form due to run-off from heavily fertilized fields (Mindat). Associated minerals include taranakite, ardealite, hydroxylapatite, variscite and gypsum (HOM).
Brushite loses all water by 100oC forming monetite (Dana).


In the Apulian caves, southern Italy, brushite is associated with hydroxylapatite and taranakite. The minerals were probably formed by interaction between phosphatic solutions derived from bat guano and calcite and clay minerals from the substratum. However, the data also suggest precipitation from solution for brushite and hydroxylapatite (AM 76.1722-1727).

The type locality is Aves Island, Nueva Esparta, Venezuela, where brushite has been found in a cave guano deposit, in druses in phosphate rock. Brushite is rather widespread in very small amoiunts in phosphate rock deposits. It typically occurs as drusy crystals in crevices in massive apatite and is a late-formed, low-temperature product. Monetite forms at higher temperatures. Brushite has been observed altered to carbonate-hydroxylapatite and to whitlockite (AM 28.218, Mindat).

At Pig Hole Cave, Giles county, Virginia, USA, brushite occurs as nodular masses of platy crystals in the lower part of a bat guano and hair deposit. The temperature of the air within the cave has been found to range from 8oC to 10oC. The cave is inhabited by a few bats and possibly small mammals or maybe owls. The cave is in a cherty limestone. A few quartz crystals are found in some passageways, and cobbles and boulders of sandstone and quartzite plus silt and clay also occur within the cave. Clusters of transparent selenite crystals occur in the clay in a few places. The material of the bottom layer of the deposit, about a foot deep, is distinctly different from that above, and it is here that the brushite occurs. The layer contains masses of black carbonaceous material, brushite and taranakite. The brushite occurs as nodular masses of minute crystals sporadically throughout this lower zone. (AM 41:616).

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