Chapmanite is a rare silicate mineral belonging to the nesosilicate group, discovered in 1924, and named in honour of the late Edward John Chapman (1821–1904), a geology professor at the University of Toronto. Chemically, it is an iron antimony silicate, closely related to bismutoferrite, and may contain aluminium impurities. It is closely associated with silver mines, most notably the Keeley mine in Ontario, Canada, found in quartz veinlets containing graphite in gneiss. It takes the form of a powdery, yellow-green, semitransparent solid, and leaves a streak of the same color. Early German texts have referred to the mineral as antimon-hypochlorite.

Chapmanite
Yellow-green earthy massive chapmanite
General
Category	Phyllosilicate
Formula (repeating unit)	Fe2Sb(SiO4)2(OH)
IMA symbol	Cpm[1]
Strunz classification	9.ED.25
Crystal system	Monoclinic
Crystal class	Domatic (m) (same H-M symbol)
Space group	Cm
Identification
Color	Yellow, green, olive green
Crystal habit	Earthy massive; granular
Cleavage	Poor
Fracture	Conchoidal to irregular
Mohs scale hardness	2.5
Luster	Nonmetallic, dull to adamantine
Streak	Yellowish green
Specific gravity	3.69-3.75
Refractive index	nα=1.850 nβ=1.950 nγ=1.960
Pleochroism	None

It was recently rediscovered in the southern hemisphere at the abandoned Argent lead mine in Bushveld series rocks of South Africa.