BH250-188
Title
BH250-188
Subject
Banded Iron Formation
Description
Major Minerals: hematite, plagioclase, quartz
Minor Minerals: muscovite, chlorite
Notes: (Thunderbird) Remnants of ooids very visible but replaced by quartz
BH250-188 is a Banded Iron Formation (BIF) sample from the Mary Ellen Mine in Biwabik, Minnesota, and represents a classic example of Precambrian iron formation. Formed approximately 1.85 to 1.9 billion years ago, this rock captures a critical moment in Earth’s history when atmospheric oxygen levels began to rise during the Great Oxidation Event. The sample exhibits the hallmark features of BIFs—alternating millimeter- to centimeter-scale bands of iron-rich and silica-rich material. The iron layers are composed primarily of hematite (Fe₂O₃) and magnetite (Fe₃O₄), with goethite (FeO(OH)) present as a secondary mineral due to weathering. The silica layers consist of chert and jasper, the latter stained red by finely dispersed hematite.
The rock’s striking red and gray banded appearance reflects alternating chemical conditions in ancient oceans. Some samples show soft-sediment deformation, and occasional oolitic textures suggest shallow marine deposition. Economically, formations like this were historically mined for iron ore and played a major role in the development of the Mesabi Range, the largest iron range in Minnesota and a major contributor to the U.S. iron industry. Scientifically, BH250-188 provides valuable insight into early ocean chemistry and the oxygenation of Earth’s atmosphere, driven by microbial life that began producing free oxygen and triggering the precipitation of dissolved iron from seawater.
Minor Minerals: muscovite, chlorite
Notes: (Thunderbird) Remnants of ooids very visible but replaced by quartz
BH250-188 is a Banded Iron Formation (BIF) sample from the Mary Ellen Mine in Biwabik, Minnesota, and represents a classic example of Precambrian iron formation. Formed approximately 1.85 to 1.9 billion years ago, this rock captures a critical moment in Earth’s history when atmospheric oxygen levels began to rise during the Great Oxidation Event. The sample exhibits the hallmark features of BIFs—alternating millimeter- to centimeter-scale bands of iron-rich and silica-rich material. The iron layers are composed primarily of hematite (Fe₂O₃) and magnetite (Fe₃O₄), with goethite (FeO(OH)) present as a secondary mineral due to weathering. The silica layers consist of chert and jasper, the latter stained red by finely dispersed hematite.
The rock’s striking red and gray banded appearance reflects alternating chemical conditions in ancient oceans. Some samples show soft-sediment deformation, and occasional oolitic textures suggest shallow marine deposition. Economically, formations like this were historically mined for iron ore and played a major role in the development of the Mesabi Range, the largest iron range in Minnesota and a major contributor to the U.S. iron industry. Scientifically, BH250-188 provides valuable insight into early ocean chemistry and the oxygenation of Earth’s atmosphere, driven by microbial life that began producing free oxygen and triggering the precipitation of dissolved iron from seawater.
Coverage
Location: Mary Ellen Mine, Biwabik, Minnesota, USA
GPS Coordinates: 47°32'6.33"N, 92°22'9.51"W
GPS Coordinates: 47°32'6.33"N, 92°22'9.51"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-188. Housed at Carleton College in Minnesota.
Type
Hand sample and thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-188,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/239.