BH250-88
Title
BH250-88
Subject
Talc-bearing phyllite
Description
Major Mineral: Talc
Minor Minerals: Fe-Ti oxides, magnetite, zircon
Optical Properties: Talc exhibits high birefringence and displays strong foliation with prominent crenulation cleavage in thin section.
Texture:
The Baraboo Quartzite is a Proterozoic metasedimentary unit, dated to Paleoproterozoic and is well-exposed in the Baraboo Range of south-central Wisconsin. In addition to quartzite, the outcrop includes other lithologies such as talc-bearing phyllite and quartz-muscovite phyllite (e.g., BH250-88, BH250-27c).
The talc-bearing phyllite represents a low-grade metamorphic with fine-grained with a well-developed foliated texture and commonly exhibits strong crenulation cleavage.
Along with talc, the rock contains Fe-Ti oxides, magnetite, and zircon as accessory phases. The occurrence of talc suggests that the protolith may have included impure dolostones, or Mg-rich shales. These lithologies likely reacted with quartz during regional metamorphism to form talc, with CO₂ released as a byproduct.
Representative Metamorphic Reaction:
Dolomite + Silica → Talc + CO₂
Minor Minerals: Fe-Ti oxides, magnetite, zircon
Optical Properties: Talc exhibits high birefringence and displays strong foliation with prominent crenulation cleavage in thin section.
Texture:
The Baraboo Quartzite is a Proterozoic metasedimentary unit, dated to Paleoproterozoic and is well-exposed in the Baraboo Range of south-central Wisconsin. In addition to quartzite, the outcrop includes other lithologies such as talc-bearing phyllite and quartz-muscovite phyllite (e.g., BH250-88, BH250-27c).
The talc-bearing phyllite represents a low-grade metamorphic with fine-grained with a well-developed foliated texture and commonly exhibits strong crenulation cleavage.
Along with talc, the rock contains Fe-Ti oxides, magnetite, and zircon as accessory phases. The occurrence of talc suggests that the protolith may have included impure dolostones, or Mg-rich shales. These lithologies likely reacted with quartz during regional metamorphism to form talc, with CO₂ released as a byproduct.
Representative Metamorphic Reaction:
Dolomite + Silica → Talc + CO₂
Coverage
Location: Baraboo, Wisconsin, USA
GPS Location: 43°30'28.08"N, 89°45'28.11"W
GPS Location: 43°30'28.08"N, 89°45'28.11"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-88. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-88,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/101.