BH250-91
Title
BH250-91
Subject
Granite
Description
Major Minerals: quartz, orthoclase, hornblende, albite, biotite
Minor Minerals: chlorite, oxides
BH250-91 is a granite sample collected from railroad gravel near Granite Falls, Minnesota, close to an open pit. Although found in gravel, it is likely out of context, having been mined from nearby outcrops. The sample is particularly valuable for teaching purposes due to the presence of biotite and hornblende—two pleochroic minerals with perfect cleavage that are often associated but distinguishable under the microscope. The biotite contains numerous zircon inclusions that exhibit radiation damage. Additionally, the orthoclase in the granite shows alteration to sericite or white mica, providing a clear example of feldspar weathering.
Minor Minerals: chlorite, oxides
BH250-91 is a granite sample collected from railroad gravel near Granite Falls, Minnesota, close to an open pit. Although found in gravel, it is likely out of context, having been mined from nearby outcrops. The sample is particularly valuable for teaching purposes due to the presence of biotite and hornblende—two pleochroic minerals with perfect cleavage that are often associated but distinguishable under the microscope. The biotite contains numerous zircon inclusions that exhibit radiation damage. Additionally, the orthoclase in the granite shows alteration to sericite or white mica, providing a clear example of feldspar weathering.
Coverage
Location: Granite Falls Railroad Quarry, Minnesota, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 91. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-91,” BH250 Mineralogy Teaching Collection, accessed April 26, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/104.