Pumpellyite is a group of closely related sorosilicate minerals: * pumpellyite-(Mg): Ca2MgAl2 * pumpellyite-(Fe2+): Ca2Fe2+Al2 * pumpellyite-(Fe3+): Ca2(Fe3+,Mg,Fe2+)(Al,Fe3+)2 * pumpellyite-(Mn2+): Ca2(Mn2+,Mg)(Al,Mn3+,Fe3+)2 * pumpellyite-(Al): Ca2(Al,Fe2+,Mg)Al2
Pumpellyite crystallizes in the monoclinic -prismatic crystal system. It typically occurs as blue-green to olive green fibrous to lamellar masses. It is translucent and glassy with a Mohs hardness of 3.2. It has refractive indices of nα=1.674–1.748, nβ=1.675–1.754 and nγ=1.688–1.764.
Pumpellyite occurs as amygdaloidal and fracture fillings in basaltic rocks in metamorphic. It is an indicator mineral of the prehnite-pumpellyite metamorphic facies. It is associated with chlorite , quartz.
It was first described in 1925 for occurrences in the Calumet mine, Keweenaw Peninisula and named for United States (1837–1923).