Segelerite is a complex phosphate mineral with formula CaMgFe³⁺OH(PO₄)₂·H₂O. It occurs in pegmatites and forms striking green or chartreuse crystals. It was discovered in 1974 in the Black Hills of South Dakota by an amateur mineralogist from New York, Curt G. Segeler (1901–1989), after whom it is named.

Segelerite
Pale yellow needle-like crystals of segelerite
General
Category	Phosphate minerals
Formula (repeating unit)	Ca2 Mg2 Fe3+2(PO4)4(OH)2 · 8H2O
IMA symbol	Sgl[1]
Crystal system	Orthorhombic
Identification
Color	Green, chartreuse, colorless, pale yellow
Cleavage	Perfect
Luster	Vitreous
Streak	White
Specific gravity	2.67

It is closely related to overite which is virtually the same mineral except that the iron is replaced by aluminium. Another mineral in the same series is juonniite wherein the iron is also replaced, this time by scandium.