Cummingtonite from the famous
Homestake Gold Mine in Lead, South Dakota.
A
close up view of the above specimen showing the plumose radiating fibers of
Cummingtonite.
CHEMISTRY - (Mg,Fe)2(Mg,Fe)5Si8O22(OH)2
CRYSTALLOGRAPHY - Monoclinic
CRYSTAL GROWTH AND HABITS - Cummingtonite occurs as bladed or columnar crystals. It commonly produces fibrous aggregates that may be plumose.
COLOR AND OPTICAL PROPERTIES - Dark gray to brown and greenish
HARDNESS - 5 - 6
SPECIFIC GRAVITY - 3.1 - 3.6
LUSTER - Vitreous
STREAK - White to light gray
BREAKABILITY - Crystals display good cleavage along {110} at angles of 54o and 126o. Cummingtonite is brittle.
OCCURRENCE - Commonly found in medium grade regionally metamorphosed rocks rich in iron. It can also be found as a late stage mineral in some mafic plutonic rocks like gabbros or norites.
ASSOCIATED MINERALS - Hornblende, actinolite, anthophyllite, tremolite, garnet.
MINERAL NAME - Cummingtonite is named after its occurrence in Cummington, Massacusetts.