Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Mg2Al3[AlSi5]O18 Fe2+ can substitute for Mg in octahedral sites, although most cordierite is fairly Mg rich. Fe3+ may substitute for Al.
Crystal System Orthorhombic (pseudohexagonal)
Crystal Habit Anhedral to to highly irregular porphyroblastic grains, less common as hexagonal prismatic crystals. Lamellar and cyclic twins common. Often uncludes numerous inclusions of quartz, opaque grains, and other minerals. Twinning more common in high-grade metamorphic rocks.

Fair cleavage on {010} and poor cleavage on {100} and {001}

Not usually obvious in hand sample or thin section. Subconchoidal fracture. Brittle.

Gray, grayish blue, blue, or indigo blue.

X = colorless, pale yellow, pale green; Y = pale blue to violet; Z= pale blue

In thin section: Colorless to very pale blue; darker with increasing Fe content. Usually no pleochrosim, though grain mounts may be distinclty pleochroic.

Lamellar and cyclic twins are common, but not visible in hand sample. The twin planes are {110} and {130}.

Twins in longitudinal section may resemble twins seen in plagioclase but thin sections may be stained to distinguish cordierite from the feldspars. Twins are more likely in cordierite from high grade metamorphic rocks than in medium-grade rocks.
Optic Sign Biaxial (+ or -) Rarely biaxial (+).
2V 35-106°

Mostly biaxial (-) with 2V between 40 and 90°. High water and Na content yield lower 2V angles.

Optic Orientation X = c, Y = a, Z = b Elongate crystals are length fast.

Refractive Indices

alpha = 1.527-1.560
beta = 1.532-1.574
gamma = 1.537-1.578
delta = 0.005-0.017

Indices of refraction increase with Fe content and also with H2O content in channels.

Max Birefringence 0.005-0.017 Low birefringence yields first-order white/gray/yellow interference colors.
Extinction Parallel   
Dispersion r < v Weak.
Distinguishing Features Resembles quartz, bur more distinctly blue. Corundum is harder. Twinned cordierite resembles plagiclase in thin section, but plagioclase has more distinct cleavage and lacks radial twinning. Yellowish, pleochroic halos around radioactive minerals such as zircon are quite common. Typically contains small, opaque inclusions. Hardness = 7.
Alteration Cordierite is readily altered to pinite, a fine-grained greenish or yellowish aggregate of chlorite, muscovite and other silicates. The alteration may develop along cracks and grain margins.

Cordierite is a common mineral in medium and high-grade pelitic metamorphic rocks. It is also common as porphyroblasts in hornfels found in contact metamorphic zones. Favored by low pressures or high temperatures. It is rare in igneous rocks, and may result form the assimilation of aluminous sediments.

Associated with chlorite, andalusite, sillimanite, kyanite, staurolite, muscovite, biotite, and chloritoid. Granite, pegmatite, gabrros, and andesite infrequently contain cordierite.

References Nesse, William D., 2000. Introduction to Mineralogy. Oxford University Press, New York, 442 p.
Editors Christina Gooch ('07), Chiza Mwinde ('18)


Photomicrograph of cordierite in plane polarized light. wollastonite in plane light. The cordierite is colorless in this thin section, with many visible inclusions. (Click to enlarge)

Thin section: RUP85T-75

Photomicrograph of cordierite in plane polarized light, rotated 90° from above picture. Notice the pleochroic yellowish halos around some of the inclusions. (Click to enlarge and to see image through crossed polars)

Photomicrograph of cordierite in plane polarized light, zoomed in to focus on a single inclusion. Click on the image to see a rollover image that shows the photomicrograph of cordierite when the stage is rotated 90 degrees. Yellowish halos form around the large inclusion and other smaller ones around it.


Youtube video showing cordierite as the microscope is being rotated.

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