Yellow crystals of the rare calcium phosphate mineral monetite from the TL in Puerto Rico (Monito Island, Mayagüez, Puerto Rico, United States of America). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Monetite
Monetite is an IMA-recognized mineral species in the phosphate class, with the chemical formula Ca(PO₃OH).
It crystallizes in the triclinic crystal system (space group P-1).
Key properties: Mohs hardness 3.5; specific gravity 2.93; Vitreous luster; optically biaxial.
IMA symbol Mnti, grandfathered status — known before the IMA’s founding, named in 1882.
Occurs as coatings and cements resulting from guano-induced phosphatization of sedimentary rocks; a coating on phosphate minerals in granite pegmatite.
It is found associated with Newberyite, gypsum, whitlockite, aphthitalite, syngenite (guano-related), apatite, lithiophilite, jahnsite, huréaulite, barbosalite, whitlockite (pegmatites).
The name refers to Moneta (now Monito) Island, which contains a notable occurrence.