Smith College - Geology 222b: Petrology
Petrographic Data File

Formula (Mg,Fe2+)2(Mg,Fe2+)5Si8O22(OH)2  
Crystal System Monoclinic Point Group: 2/m
Cleavage Perfect on {110} Intersects at 56 and 124
Twinning Simple or multiple Parallel to {100}, prevalence of fine polysynthetic twinning
Streak White Brittle
Luster Vitreous, silky  
Hardness 5-6  
Crystal Habit Bladed, columnar, or fibrous crystals and granular aggregates Grow up to 20 cm, elongate parallel to c-axis. Forms fibers, plates, and needles.

Colorless to pale green or pale brown in thin section

Darker colors in thin section result from higher Fe content. May be translucent to non-transparent

X = colorless

Y = colorless

Z = pale green

Weak pleochroism, found primarily in samples with higher Fe content
Optic Sign Biaxial (+) 

Measured: 70° - 90

2V varies sytematically with Mg content.

Optic Orientation

X^a= -9 to -3°


Z^c= -21° to -16°

The Z^c extinction angle varies fairly systematically with composition.

Refractive Indices

alpha = 1.633-1.663

beta = 1.638-1.677

gamma = 1.655-1.697

Indicies of refraction decrease systematically with Mg content
Maximum Birefringence 0.020-0.037 Birefringesnce decreases systematcally with Mg content
Elongation Length slow
Extinction Inclined
Dispersion r < v Weak
Mineral Group Amphibole
Distinguishing Features

Hand Sample: Distinguished as an amphibole by habit and cleavage. Brown Varieties closly resemble orthoamphibole and green varieties closely resemble the calcic clinoampiboles - optical tests are required for definite identification.

Thin section: Cummingtonite is differentiated by its inclined extinction, higher birefringence, high relief, and prevalance of fine polysynthetic twinning.


Medium-grade regionally metamorphosed mafic and ultramafic rocks; characteristic of metamorphosed iron formations. Cummingtonite is a late-stage mineral in some intermediate volcanic rocks, primarily gabbros and norites. Rarely found in silicic volcanic rocks.

Associated minerals: tremolite, gedrite, garnet, magnetite, chlorite, plagioclase, hornblend, actinolite, quartz, and biotite.

Editors Alyssa Doody (06), Monica Rolls (13)

-Handbook of Mineralogy, 2001, Mineralogical Society of America, cummingtonite.pdf
-Duda, Rudolph and Lubos Rejl, 1989. Minerals of the World, Arch Cape Press, New York, 520p.
-Nesse, William D., 2000. Introduction to Mineralogy, Oxford University Press, New York, 442 p.

Photomicrograph of cummingtonite in plain light, click to view larger image. 10x magnification. Note light brown pleochorism.
Photomicrograph of cummingtonite in plain light, click to view larger image. 10x magnification. 
Photomicrograph of cummingtonite in plain light, click to view larger image. 10x magnification.
Photomicrograph of cummingtonite in plane light. Mag. 10x.
Photomicrograph of cummingtonite in plane light. Click to view larger image. Mag. 5x.

Return to Petrography Index