Cuspidine

Cuspidine
Yellowish cuspidine crystals from the type locality at Mount Vesuvius
General
Category	Sorosilicate
Formula	Ca4(Si2O7)(F,OH)2
IMA symbol	Csp
Strunz classification	9.BE.17
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 10.93 Å, b = 10.57 Å, c = 7.57 Å; β = 110.11°; Z=4
Identification
Color	Colorless, tan, light brown, pale red
Crystal habit	Minute spearhead-shaped crystals, acicular, granular
Twinning	Simple, lamellar, polysynthetic on {100}
Cleavage	Good on {001} imperfect on {110}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Diaphaneity	Transparent to translucent
Specific gravity	2.85–2.96
Optical properties	Biaxial (+)
Refractive index	nα = 1.586 – 1.594 nβ = 1.589 – 1.596 nγ = 1.598 – 1.606
Birefringence	δ = 0.012
2V angle	Measured: 59° to 71°
References

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.