BH250-246
Title
BH250-246
Subject
Garnet-Staurolite-Cummingtonite Schist
Description
Major Minerals: cummingtonite, garnet, staurolite
Minor Minerals: quartz
Mineralogy Comment: cummingtonite crystals are ~2.5 cm long
Texture: prismatic, bladed, fibrous, solid solution series with grunerite
This garnet–staurolite–cummingtonite schist from the Upper Peninsula of Michigan is a striking medium- to high-grade metamorphic rock defined by exceptionally long, prismatic crystals of cummingtonite, reaching up to ~2.5 cm in length. These amphibole crystals are unusually well developed—appearing elongate, bladed, and locally fibrous in both hand sample and thin section—making them among the most impressive examples of cummingtonite growth. The rock also contains garnet and staurolite, minerals characteristic of intermediate to higher-grade metamorphism, with minor quartz forming part of the matrix. The strong mineral alignment and elongate habit of cummingtonite contribute to a pronounced schistosity, reflecting deformation during metamorphism. Mineralogically, cummingtonite belongs to a solid solution series with grunerite, indicating variable Fe–Mg compositions and recording changing metamorphic conditions. This assemblage suggests metamorphism of an iron- and magnesium-rich protolith, likely under conditions where both temperature and fluid activity facilitated the growth of large amphibole crystals. Overall, the rock provides an excellent example of how mineral growth, deformation, and chemical variation interact to produce distinctive textures and mineral assemblages in metamorphic environments.
Minor Minerals: quartz
Mineralogy Comment: cummingtonite crystals are ~2.5 cm long
Texture: prismatic, bladed, fibrous, solid solution series with grunerite
This garnet–staurolite–cummingtonite schist from the Upper Peninsula of Michigan is a striking medium- to high-grade metamorphic rock defined by exceptionally long, prismatic crystals of cummingtonite, reaching up to ~2.5 cm in length. These amphibole crystals are unusually well developed—appearing elongate, bladed, and locally fibrous in both hand sample and thin section—making them among the most impressive examples of cummingtonite growth. The rock also contains garnet and staurolite, minerals characteristic of intermediate to higher-grade metamorphism, with minor quartz forming part of the matrix. The strong mineral alignment and elongate habit of cummingtonite contribute to a pronounced schistosity, reflecting deformation during metamorphism. Mineralogically, cummingtonite belongs to a solid solution series with grunerite, indicating variable Fe–Mg compositions and recording changing metamorphic conditions. This assemblage suggests metamorphism of an iron- and magnesium-rich protolith, likely under conditions where both temperature and fluid activity facilitated the growth of large amphibole crystals. Overall, the rock provides an excellent example of how mineral growth, deformation, and chemical variation interact to produce distinctive textures and mineral assemblages in metamorphic environments.
Coverage
Upper peninsula, Michigan, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-246. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-246,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/326.