Pumpellyite-(Mg)

pumpellyite-(Mg)

glaucophane

epidote

prehnite

Images

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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