Glaucophane
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Formula | Na2Mg3Al2Si8O22(OH)2 |
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. |
Color/Pleochroism | 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 = |
1.606-1.701 1.662-1.711 1.627-1.717 0.006-0.029 |
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) |