BH250-145b
Title
BH250-145b
Subject
Granite
Description
Major Minerals: orthoclase, plagioclase, biotite, muscovite
Mineralogy Comments: myrmekite
BH250-145b is a large, rounded granitic glacial erratic located in the Carleton College Arboretum. Its smoothly curved surface reflects abrasion and weathering during transport by glacial ice. Because granite does not occur as local bedrock at this site, the boulder was transported southward from a Precambrian source area, probably in central or northern Minnesota or the Canadian Shield. Its precise provenance is currently unknown.
The rock contains abundant orthoclase and plagioclase accompanied by biotite and muscovite. Thin sections reveal a coarse, interlocking crystalline texture and myrmekite—fine, wormlike intergrowths of quartz within or along the margins of plagioclase. Myrmekite commonly develops during late-stage crystallization or subsequent mineral replacement and deformation in granitic rocks.
Thin sections are available for additional petrographic study. No whole-rock chemical analysis is currently available. Mineral chemistry, whole-rock major- and trace-element analysis, or zircon U–Pb dating could help constrain the age and source of the erratic.
Mineralogy Comments: myrmekite
BH250-145b is a large, rounded granitic glacial erratic located in the Carleton College Arboretum. Its smoothly curved surface reflects abrasion and weathering during transport by glacial ice. Because granite does not occur as local bedrock at this site, the boulder was transported southward from a Precambrian source area, probably in central or northern Minnesota or the Canadian Shield. Its precise provenance is currently unknown.
The rock contains abundant orthoclase and plagioclase accompanied by biotite and muscovite. Thin sections reveal a coarse, interlocking crystalline texture and myrmekite—fine, wormlike intergrowths of quartz within or along the margins of plagioclase. Myrmekite commonly develops during late-stage crystallization or subsequent mineral replacement and deformation in granitic rocks.
Thin sections are available for additional petrographic study. No whole-rock chemical analysis is currently available. Mineral chemistry, whole-rock major- and trace-element analysis, or zircon U–Pb dating could help constrain the age and source of the erratic.
Coverage
Location: Carleton College Arboretum, Northfield, Minnesota, USA
GPS Coordinates: 44°28'2.97"N, 93° 8'41.89"W
GPS Coordinates: 44°28'2.97"N, 93° 8'41.89"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 145b. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-145b,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/176.