Cuspidine is a fluorine-bearing calcium sorosilicate mineral with the formula Ca4(Si2O7)(F,OH)2. It crystallises in the monoclinic crystal system and is a member of the wöhlerite group.
History and classification Cuspidine was first described in 1876 from Monte Somma, Italy. Its name derives from the Latin cuspis, meaning “spear”, in reference to its characteristic spear-shaped crystals. It was the first mineral now assigned to the wöhlerite group whose crystal structure was determined; consequently, older literature sometimes called the association the “cuspidine group”. The International Mineralogical Association formally established the wöhlerite group in 2020, using the name of wöhlerite because that mineral had been described earlier.
Structure and properties The structure contains four-column-wide walls of cation-centred polyhedra linked by corner sharing and by Si2O7 groups. Cuspidine is monoclinic, with space group P21/c. It typically forms minute acicular or spear-shaped crystals and granular aggregates, ranging from colourless and tan to pale red or light brown. Its Mohs hardness is 5–6 and its specific gravity is about 2.85–2.96.
Occurrence At Monte Somma, cuspidine occurs in volcanic tuff. In the Franklin, New Jersey, mining district it occurs in contact-metamorphosed limestone, whereas at Dupezeh Mountain in Iraq it occurs in melilite-bearing skarn. Reported associated minerals include augite, hornblende, diopside, grossular, biotite, phlogopite, monticellite, wollastonite, calcite, spinel, magnetite and perovskite.
References