Formula: Ca(PO3OH)
Anhydrous acid phosphate
Specific gravity: 2.929
Hardness: 3½
Streak: White
Colour: White, yellow or colourless, colorless in transmitted light

Cave deposits

Monetite occurs as coatings and cements resulting from guano-induced phosphatisation of sedimentary rocks, where it is associated with newberyite, gypsum, whitlockite, aphthitalite and syngenite. Monetite also occurs as a coating on phosphate minerals in granite pegmatites, associated with apatite, lithiophilite, jahnsite, hureaulite, barbosalite and whitlockite. Powdered monetite takes up water from the atmosphere and forms brushite (Mindat).


At the Niah Great Cave, Miri Division, Sarawak, Malaysia, monetite occurs in a cave deposit of phosphates (Dana).

At the Cioclovina cave, Boșorod, Hunedoara, Hunedoara, Romania, the common cave-minerals calcite, aragonite, gypsum, hydroxylapatite, brushite, monetite, crandallite and taranakite are accompanied by a number of rare mineral species such as tinsleyite, sampleite, paratacamite or clinoatacamite, churchite-(Y) and foggite (CM 47.207-208).

At the Măgurici Cave, Sălaj, Romania, the phosphatisation of illite and montmorillonite clay rich sediments, located on the cave's floor, lead to the formation of taranakite and francoanellite. Mineral assemblages precipitated at the boundary between limestone bedrock and guano deposits are dominantly hydroxylapatite, brushite, ardealite and monetite. A number of sulphate minerals such as gypsum, bassanite, mirabilite and cesanite were precipitated along with the phosphates. The major parameters controlling the environments under which these mineral assemblages were deposited are pH, relative humidity, alkali content and Ca/P ratio (EJM 15.741-745).

At Ascension Island, Saint Helena, Ascension and Tristan da Cunha, monetite occurs with apatite and newberyite (Dana).

At Mona Island, Mayagüez, Puerto Rico, USA, monetite occurs with gypsum (Dana).

At the type locality, Monito Island, Mayagüez, Puerto Rico, USA, monetite is formed by the interaction of bird guano in a limestone cave (Mindat).

At the Los Monges Islands, Federal Dependencies, Venezuela, monetite occurs with whitlockite (Dana).

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