cummingtonite

{{Infobox mineral | name = Cummingtonite | category = Inosilicate | boxwidth = | boxbgcolor = | image = Amphibole - Cummingtonite w- chlorite in schist Magnesium iron silicate 3800 foot level Homestake Mine Lawrence COunty South Dakota 2071.jpg | caption = | formula = | IMAsymbol=Cum | molweight = | strunz = 9.DE.05 | system = Monoclinic | class = Prismatic (2/m) H-M symbol: (2/m) | symmetry = Monoclinic Space group: C2/m | unit cell = a = 9.53 Å, b = 18.23 Å, c = 5.32 Å; β = 101.97°; Z = 2 | color = Dark green, brown, gray, beige; colorless to pale green in thin section | habit = Rarely as distinct crystals. Columnar to fibrous and granular | twinning = Simple and lamellar – common | cleavage = Good on {110} intersecting at 54 and 126° | fracture = Splintery | tenacity = Brittle | mohs = 5–6 | luster = Vitreous to silky | refractive = nα = 1.639–1.671 nβ = 1.647–1.689 nγ = 1.664–1.708 | opticalprop = Biaxial (+) | birefringence = δ = 0.025–0.037 | pleochroism = With increasing iron content, weak; X = Y = colorless; Z = pale green | 2V = Measured: 65° to 90°, Calculated: 70° to 90° | streak = | gravity = 3.1–3.6 | density = | melt = | fusibility = | diagnostic = Characterized by light brown color and needlelike, often radiating habit. Difficult to distinguish from anthophyllite or gedrite without optical and/or X-ray tests. | solubility = | diaphaneity = Translucent, will transmit light on thin edges. | other = |references = }}

Cummingtonite ( ) is a metamorphic amphibole with the chemical composition , magnesium iron silicate hydroxide.