BH250-23
Title
BH250-23
Subject
Glaucophane Schist
Description
Major Mineral: Glaucophane
Description
BH250-23 is a fine-grained, foliated metamorphic rock characterized by a blue to lavender hue due to its high content of glaucophane, a sodic amphibole that forms under high-pressure, low-temperature conditions typical of blueschist facies metamorphism. This rock is a textbook example of subduction zone metamorphism.
Mineralogy: Glaucophane is the dominant mineral; occurs as euhedral to subhedral crystals with distinctive blue to lavender pleochroism. It is a sodic amphibole, and its presence marks high-pressure conditions. Na-bearing amphibole: General term likely referring to the compositional variety among amphiboles that includes glaucophane.
Quartz: Present in the matrix, contributing to the rock’s overall strength and interlocking texture.
Texture: Radiating glaucophane crystals: Glaucophane forms acicular to prismatic crystals that radiate from growth centers, giving a characteristic texture.
Porphyroblastic texture: Some glaucophane crystals may grow larger than the matrix, forming porphyroblasts.
Foliation: Defined by alignment of elongated glaucophane crystals and quartz ribbons.
Optical Properties:
Pleochroism: Glaucophane displays strong pleochroism, typically:
Cleavage: Perfect in two directions at 60° and 120°, typical of amphiboles. Accouple of movies that show this cleavage sets are available.
Glaucophane schist belongs to the blueschist facies, diagnostic of subduction zones where oceanic crust and sediments are buried rapidly and metamorphosed under high pressure, low temperature. Common in accretionary prisms and subduction complexes.
Description
BH250-23 is a fine-grained, foliated metamorphic rock characterized by a blue to lavender hue due to its high content of glaucophane, a sodic amphibole that forms under high-pressure, low-temperature conditions typical of blueschist facies metamorphism. This rock is a textbook example of subduction zone metamorphism.
Mineralogy: Glaucophane is the dominant mineral; occurs as euhedral to subhedral crystals with distinctive blue to lavender pleochroism. It is a sodic amphibole, and its presence marks high-pressure conditions. Na-bearing amphibole: General term likely referring to the compositional variety among amphiboles that includes glaucophane.
Quartz: Present in the matrix, contributing to the rock’s overall strength and interlocking texture.
Texture: Radiating glaucophane crystals: Glaucophane forms acicular to prismatic crystals that radiate from growth centers, giving a characteristic texture.
Porphyroblastic texture: Some glaucophane crystals may grow larger than the matrix, forming porphyroblasts.
Foliation: Defined by alignment of elongated glaucophane crystals and quartz ribbons.
Optical Properties:
Pleochroism: Glaucophane displays strong pleochroism, typically:
Cleavage: Perfect in two directions at 60° and 120°, typical of amphiboles. Accouple of movies that show this cleavage sets are available.
Glaucophane schist belongs to the blueschist facies, diagnostic of subduction zones where oceanic crust and sediments are buried rapidly and metamorphosed under high pressure, low temperature. Common in accretionary prisms and subduction complexes.
Coverage
Okland Californian
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-23. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-23,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/26.