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Glaucophane

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The Glaucophane mineral is named after two Greek words: glaukos, which means “bluish green”; and phainesthai, which means “to appear.” Specimens can be gray, lavender blue, or bluish black. Crystals are slender, often lathlike prisms, with lengthwise striations. Twinning is common. Glaucophane can also be massive, fibrous, or granular. When iron replaces the magnesium in its structure, it is known as ferroglaucophane. Glaucophane occurs in schists formed by high-pressure metamorphism of sodium-rich sediments at low temperatures (up to 400°F/200°C) or by the introduction of sodium into the process. Glaucophane is often accompanied by jadeite, epidote, almandine, and chlorite. It is one of the minerals that are referred to as asbestos. Glaucophane and its associated minerals are known as the glaucophane metamorphic facies. The presence of these minerals indicates the range of temperatures and pressures under which metamorphism occurs.

Name: From the Greek for bluish green and to appear.

Polymorphism & Series: Forms a series with ferroglaucophane.

Mineral Group: Amphibole (alkali) group: Fe 2+=(Fe 2+ + Mg) < 0.5; Fe 3+=(Fe 3+ + Al vi ) < 0.3; (Na + K)A < 0.5; NaB ¸ 1.34.

Association: Crossite, chlorite, epidote, pumpellyite, lawsonite, omphacite, jadeite, actinolite, barroisite, cummingtonite, aragonite.

Chemical Properties of Glaucophane

Chemical Classification Inosilicates
Chemical Composition Na2(Mg3Al2)Si8O22(OH)

Physical Properties of Glaucophane

Color Grey to lavender-blue.
Streak Pale grey to bluish-grey.
Luster Vitreous
Cleavage Good on [110] and on [001]
Diaphaneity Translucent
Mohs Hardness 5 – 6 on Mohs scale
Diagnostic Properties 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.
Crystal System Monoclinic
Fracture Brittle – conchoidal
Density 3 – 3.15

Optical Properties of Glaucophane

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
Optical Extinction  
2V: Measured: 10° to 80°, Calculated: 62° to 84°
RI values: nα = 1.606 – 1.637 nβ = 1.615 – 1.650 nγ = 1.627 – 1.655
Optic Sign Biaxial (-)
Birefringence δ = 0.021
Relief Moderate
Dispersion: strong

Occurrence

Characteristic of the blueschist facies, in former subduction zones in mountain belts; in the greenschist facies and in eclogites that have undergone retrograde metamorphism.

Distribution

Widespread in some mountain belts. On Syra Island, Cyclades Islands, Greece. At numerous sites in the California Coast Ranges, as on the Tiburon Peninsula and at Vonsen Ranch, Marin Co., at Glaucophane Ridge, Panoche Valley, San Benito Co., and near Valley Ford, Sonoma Co.; in the Kodiak Islands, Alaska, USA. At St. Marcel, Val d’Aosta, and Piollore (Biella), Piedmont, Italy. On Anglesey, Wales. In Japan, at Ubuzan, Aichi Prefecture, and Otakiyama, Tokushima Prefecture.

References

Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.

Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].

Mindat.org. (2019). Halite: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].

Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].

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