BH250-46
Title
Subject
Description
Minor Minerals: plagioclase, sericite
Mineralogy Comments: Carlsbad twinning, sericite secondary alteration
BH250-46 is an anorthosite sample from near Silver Bay, Minnesota, and is part of the Duluth Complex, a massive Mesoproterozoic-age intrusive body. Like other anorthosites in the complex, this rock is notable for its plagioclase-rich composition. The dominant mineral is plagioclase feldspar, typically ranging from labradorite to bytownite (An₅₀–An₈₀), comprising over 90% of the rock. Plagioclase crystals are generally euhedral to subhedral, often centimeter-scale, and exhibit weak to moderate zoning.
The bulk composition is high in Al₂O₃ due to plagioclase dominance, and low in SiO₂ compared to granitic rocks (typically ~50–55 wt%). K₂O and Na₂O contents are also low, consistent with the presence of calcic plagioclase.
The rock exhibits a typical cumulate texture, and is found interlayered with troctolite, gabbro, and oxide-rich layers, appearing here as a large xenolith, as shown in the accompanying photograph.
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