BH250-66
Title
BH250-66
Subject
Sillimanite Schist
Description
Major Minerals: biotite, muscovite, quartz, sillimanite
Mineralogy Comments: biotite interwoven with muscovite and quartz
BH250-66 is mapped as a sillimanite schist and was collected from the Black Hills of South Dakota, in the same metamorphic terrane as BH250-67 and BH250-68. While the rock is mapped as containing sillimanite, it is difficult to identify in the field, and even in thin section, the sillimanite grains are sparse and poorly developed. Some grains resemble sillimanite, but large, definitive crystals are hard to find. Despite this, the sample contains an abundance of well-aligned biotite and muscovite, creating a strongly foliated texture indicative of regional metamorphism. The schistosity is well-developed, and the quartz grains—though somewhat variable in size—exhibit undulose extinction, pointing to dynamic recrystallization. Several biotite grains host zircon inclusions with pleochroic halos, features commonly used in optical mineralogy to trace metamorphic processes. While sillimanite is difficult to confirm, the sample remains an excellent resource for teaching metamorphic textures, mineral identification challenges, and the interpretation of regional metamorphic conditions.
Mineralogy Comments: biotite interwoven with muscovite and quartz
BH250-66 is mapped as a sillimanite schist and was collected from the Black Hills of South Dakota, in the same metamorphic terrane as BH250-67 and BH250-68. While the rock is mapped as containing sillimanite, it is difficult to identify in the field, and even in thin section, the sillimanite grains are sparse and poorly developed. Some grains resemble sillimanite, but large, definitive crystals are hard to find. Despite this, the sample contains an abundance of well-aligned biotite and muscovite, creating a strongly foliated texture indicative of regional metamorphism. The schistosity is well-developed, and the quartz grains—though somewhat variable in size—exhibit undulose extinction, pointing to dynamic recrystallization. Several biotite grains host zircon inclusions with pleochroic halos, features commonly used in optical mineralogy to trace metamorphic processes. While sillimanite is difficult to confirm, the sample remains an excellent resource for teaching metamorphic textures, mineral identification challenges, and the interpretation of regional metamorphic conditions.
Coverage
Location: Black Hills, South Dakota, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 66. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-66,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/76.