BH250-27c
Title
BH250-27c
Subject
Baraboo Quartzite-phylletic Subunit
Description
Major Minerals: quartz, muscovite, iron oxides
Minor Mineral: zircon
BH250-27c represents the phyllitic facies of the Baraboo Quartzite and is distinctive from the other units due to its fine-grained and foliated texture. There are bands of foliation that show some of the original bedding plane.
Mineralogy:
Fine to medium grained quartz, strained, and recrystallized. Quartz grains commonly display undulose extinction under cross-polarized light, reflecting metamorphic deformation.
Fine grained muscovite aligned along foliation planes, giving the rock the characteristic silky sheen of phyllite.
Sericite is present as a fine grained mica, commonly derived from the alteration of feldspar and clay minerals; contributes to the rock's cleavage development.
Many iron oxides (hematite and magnetite), occurring as disseminated grains or fine streaks.
Small grains of zircon are present as detrital grains; these minerals are stable under metamorphic conditions and typically resistant to alteration.
Protolith:
Based on its mineralogy and texture, the protolith of BH250-27c was likely a fine-grained arkosic to feldspathic sandstone, siltstone, or mudstone., deposited in a nearshore marine to deltaic or fluvial environment. The presence of feldspar and clay-rich sediments that are now muscovite and sericite, suggests a proximal source, calm environment possibly derived from erosion of nearby source rock basin development.
Minor Mineral: zircon
BH250-27c represents the phyllitic facies of the Baraboo Quartzite and is distinctive from the other units due to its fine-grained and foliated texture. There are bands of foliation that show some of the original bedding plane.
Mineralogy:
Fine to medium grained quartz, strained, and recrystallized. Quartz grains commonly display undulose extinction under cross-polarized light, reflecting metamorphic deformation.
Fine grained muscovite aligned along foliation planes, giving the rock the characteristic silky sheen of phyllite.
Sericite is present as a fine grained mica, commonly derived from the alteration of feldspar and clay minerals; contributes to the rock's cleavage development.
Many iron oxides (hematite and magnetite), occurring as disseminated grains or fine streaks.
Small grains of zircon are present as detrital grains; these minerals are stable under metamorphic conditions and typically resistant to alteration.
Protolith:
Based on its mineralogy and texture, the protolith of BH250-27c was likely a fine-grained arkosic to feldspathic sandstone, siltstone, or mudstone., deposited in a nearshore marine to deltaic or fluvial environment. The presence of feldspar and clay-rich sediments that are now muscovite and sericite, suggests a proximal source, calm environment possibly derived from erosion of nearby source rock basin development.
Coverage
Location: Baraboo, Wisconsin, USA
GPS Coordinates: 43.435519, -89.773492
GPS Coordinates: 43.435519, -89.773492
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-27c. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-27c,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/320.