BH250-191
Title
BH250-191
Subject
Anorthosite
Description
Major Minerals: anorthite, orthopyroxene, plagioclase
Minor Minerals: serpentine
BH250-191 is an anorthosite sample collected near Silver Bay, Minnesota, and is part of the Mesoproterozoic Duluth Complex—a massive intrusive body. Like other anorthosites in the complex, this rock is characterized by its plagioclase-rich composition. The dominant mineral is plagioclase feldspar, typically ranging from labradorite to bytownite (An₅₀–An₈₀), comprising over 90% of the rock. The plagioclase crystals are generally euhedral to subhedral, commonly centimeter-scale, and exhibit weak to moderate zoning.
BH250-191 differs from BH250-46 in that its plagioclase shows some undulose extinction, likely due to thermal heating from the intrusion of the Duluth gabbro. The sample is located near BH250-193, which marks the contact between gabbro and anorthosite. Several student projects based on BH250-193 explore the history of this contact and the effects of thermal alteration.
The bulk composition of BH250-191 is high in Al₂O₃, reflecting the abundance of calcic plagioclase, and relatively low in SiO₂ (~50–55 wt%) compared to granitic rocks. K₂O and Na₂O contents are also low, consistent with the dominance of calcium-rich feldspar.
The rock displays a typical cumulate texture and occurs interlayered with troctolite, gabbro, and oxide-rich layers. In this location, the anorthosite appears as a large xenolith, as seen in the accompanying photograph.
Minor Minerals: serpentine
BH250-191 is an anorthosite sample collected near Silver Bay, Minnesota, and is part of the Mesoproterozoic Duluth Complex—a massive intrusive body. Like other anorthosites in the complex, this rock is characterized by its plagioclase-rich composition. The dominant mineral is plagioclase feldspar, typically ranging from labradorite to bytownite (An₅₀–An₈₀), comprising over 90% of the rock. The plagioclase crystals are generally euhedral to subhedral, commonly centimeter-scale, and exhibit weak to moderate zoning.
BH250-191 differs from BH250-46 in that its plagioclase shows some undulose extinction, likely due to thermal heating from the intrusion of the Duluth gabbro. The sample is located near BH250-193, which marks the contact between gabbro and anorthosite. Several student projects based on BH250-193 explore the history of this contact and the effects of thermal alteration.
The bulk composition of BH250-191 is high in Al₂O₃, reflecting the abundance of calcic plagioclase, and relatively low in SiO₂ (~50–55 wt%) compared to granitic rocks. K₂O and Na₂O contents are also low, consistent with the dominance of calcium-rich feldspar.
The rock displays a typical cumulate texture and occurs interlayered with troctolite, gabbro, and oxide-rich layers. In this location, the anorthosite appears as a large xenolith, as seen in the accompanying photograph.
Coverage
Location: Northern Minnesota, Minnesota, USA
Date
Fall 2014
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-164. Housed at Carleton College in Minnesota.
Contributor
George and Dean
Type
Hand sample and thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-191,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/248.