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Formula: Ca2MgAl2(Si2O7)(SiO4)(OH,O)2.H2O
Sorosilicate (Si2O7 groups),
pumpellyite subgroup of
the pumpellyite group, forms a series with
julgoldite-(Fe2+)
Crystal System: Monoclinic
Specific gravity: 3.18 measured, 3.20 calculated
Hardness: 5½
Streak: White
Colour: Green, blue, blackish green, brown
Environments
Metamorphic environments
Basaltic cavities
Pumpellyite-(Mg) is a widespread product of low-grade metamorphism of amygdaloidal
basalts and dolerite; in
glaucophane schist and in
graywacke. Associated minerals include
chlorite, epidote,
actinolite, lawsonite,
glaucophane, prehnite,
zeolites and quartz
(HOM).
Localities
Keweenaw county, Michigan, USA, is the type locality for pumpellyite-(Mg).
At the Central Mine, Central, Keweenaw county, Michigan, USA, pumpellyite-(Mg), better known as the variety
chlorastrolite or “greenstone,” is a fine-grained, radial form of the species which, when polished, exhibits attractive
chatoyancy and sector zoning, resulting in a segmented pattern sometimes referred to as “turtleback.” This lapidary
material has become a prized Copper Country specialty, and chlorastrolite is the state gemstone of Michigan. The Central
mine is probably the best locality for this material, that occurs as amgydules in
basalt, typically 1 to 5 cm wide
(MinRec 79-80).
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