Smith College - Geology 222b - Petrology

Petrographic Data File



Principle substitutions are Fe2+ for Mg and Fe3+ for Al (total subsitution produces Riebeckite, partial substituion produces Crossite)
Crystal System Monoclinic 2/m
Crystal Habit Bladed to slender prismatic crystals Also fibrous or granular
Cleavage {110} perfect; intersects at 56 and 124° Parting on {100}, conchoidal to uneven fracture. Brittle.

Lavender blue, blue, dark blue, gray or black.

Distinct pleochroism: X= colorless, pale blue, yellow; Y= lavender-blue, bluish green; Z= blue, greenish blue, violet

Darker with increasing Fe content. Light blue in thin section.

Blue in color under regular lighting, and often found in differing shades of blue.

Optic Sign Biaxial (+ or -) Usually negative
2V 10-45° .
Optic Orientation
X= a
Y= c
Z= b
O.A.P. = (010)


Varies with Fe content.

Refractive Indices alpha = 
beta = 
gamma = 
delta = 

Increase with Fe content.
Max Birefringence 0.006-0.023 Varies with Fe content.
Dispersion r < v Weak
Distinguishing Features Distinguished from other amphiboles by distinct blue color in hand sample. Blue pleochroism in thin section/grain mount distinguishes from other amphiboles. Glaucophane has length slow, riebeckite length fast. Darkest when c-axis parallel to vibration direction of lower polarizer (blue tourmaline is darkest w/ c-axis perpendicular to vibration direction of polarizer). There is no twinning in glaucophane. Glaucophane also has a parallel extinction when viewed under cross polars.
Occurrence Characteristic of high-pressure, low-temperature regional metamorphic rocks (blue schist). Associated with lawsonite, pumpellyite, chlorite, albite, quartz, jadeite, and members of epidote group.
Editors Amanda Nyren (06), Bridget Mahoney (MHC '06), Camille Dywer ('14)

Photomicrograph in plane polarized light showing lavender blue pleochroism.
Photomicrograph in plane polarized light with the stage rotated 90 degrees from above picture.
Photomicrograph of glaucophane in plain polarized light. Note the varying shades of blue that are observed in plain polarized light. Roll over image is the same photomicrograph in cross polarized light.
To the left is a photomicrograph of glaucophane, the lavender colored mineral. When the stage is rotated 90 degress, glaucophane's color changes from lavender to a clear, whitish purple. Click on the image to see a pleochroism rollover.

To the left is a photomicrograph of glaucophane. Click on the image to see a larger image with a rollover of cross polars.

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