BH250-26
Title
BH250-26
Subject
Garnet Staurolite Schist
Description
Major Minerals: garnet, staurolite, muscovite, biotite, sillimanite
Minor Minerals: tourmaline, chlorite
Mineralogy Comments: good muscovite and staurolite; small tourmaline
Texture: foliation, optic axis figure on quartz, inclusions in garnet
Sample BH250-26 likely derives from a pelitic protolith, a fine-grained mudstone or shale, rich in Al, K, and Fe. This is supported by the presence of aluminosilicate minerals (garnet, staurolite, sillimanite) and abundant muscovite and biotite, typical of K- and Al-rich sediments. Trace tourmaline, either detrital or metamorphic, further supports a metasedimentary origin.
Metamorphic Mineral Assemblage:
Major: Garnet + Staurolite + Muscovite + Biotite + Sillimanite
(Make thin sections for SEM work to confirm whether inclusions within muscovite are sillimanite.)
Minor: Tourmaline + Chlorite
This assemblage reflects Barrovian-type regional metamorphism within the upper amphibolite facies.
Chlorite: Likely formed during retrograde metamorphism from biotite or garnet.
Tourmaline: May be detrital or have formed from boron-rich fluids during prograde or retrograde phases.
Texture and Structural Features:
Foliation: Strong, formed by alignment of muscovite and biotite under ductile deformation during regional compression.
Garnet Inclusions: Suggest pre-kinematic garnet growth, with “snowball textures” recording early foliation and mineral assemblages.
Significance of Staurolite:
Staurolite: An index mineral for intermediate-grade conditions. Its coexistence with sillimanite marks a transition zone and suggests breakdown reactions such as.
Minor Minerals: tourmaline, chlorite
Mineralogy Comments: good muscovite and staurolite; small tourmaline
Texture: foliation, optic axis figure on quartz, inclusions in garnet
Sample BH250-26 likely derives from a pelitic protolith, a fine-grained mudstone or shale, rich in Al, K, and Fe. This is supported by the presence of aluminosilicate minerals (garnet, staurolite, sillimanite) and abundant muscovite and biotite, typical of K- and Al-rich sediments. Trace tourmaline, either detrital or metamorphic, further supports a metasedimentary origin.
Metamorphic Mineral Assemblage:
Major: Garnet + Staurolite + Muscovite + Biotite + Sillimanite
(Make thin sections for SEM work to confirm whether inclusions within muscovite are sillimanite.)
Minor: Tourmaline + Chlorite
This assemblage reflects Barrovian-type regional metamorphism within the upper amphibolite facies.
Chlorite: Likely formed during retrograde metamorphism from biotite or garnet.
Tourmaline: May be detrital or have formed from boron-rich fluids during prograde or retrograde phases.
Texture and Structural Features:
Foliation: Strong, formed by alignment of muscovite and biotite under ductile deformation during regional compression.
Garnet Inclusions: Suggest pre-kinematic garnet growth, with “snowball textures” recording early foliation and mineral assemblages.
Significance of Staurolite:
Staurolite: An index mineral for intermediate-grade conditions. Its coexistence with sillimanite marks a transition zone and suggests breakdown reactions such as.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-26. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-26,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/29.