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Category Sorosilicate
(repeating unit)
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
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 [1][2][3]

Cuspidine is a fluorine bearing calcium silicate mineral (sorosilicate) with formula: Ca4(Si2O7)(F,OH)2.[1] Cuspidine crystallizes in the monoclinic crystal system and occurs as acicular to spear shaped pale red to light brown crystals. It is a member of the wöhlerite group.

Cuspidine was first described in 1876 for an occurrence in Monte Somma, Italy.[1][3] The name is from the Greek cuspis for spear from its characteristic crystal form.[1] Cuspidine occurs as crystals in tuff from Monte Somma. In the Franklin, New Jersey mine area it occurs in contact metamorphosed limestone. In Dupezeh Mountain, Iraq, it occurs in melilite bearing skarn. Associated minerals include augite, hornblende, diopside, grossular, biotite, phlogopite, monticellite, wollastonite, calcite, spinel, magnetite and perovskite.[3]