BH250-88
Title
BH250-88
Subject
Talc-bearing phyllite
Description
Major Mineral: Talc
Minor Minerals: Fe-Ti oxides, magnetite, zircon
BH250-88 represents one of the most unusual lithologies preserved within the otherwise quartz-dominated Baraboo Quartzite succession. Deposited approximately 1.7 billion years ago, the Baraboo Quartzite primarily formed in a high-energy shoreline system where repeated wave action and sediment reworking produced exceptionally mature quartz sand. Rare lithologies such as talc-bearing phyllite and quartz-muscovite phyllite (e.g., BH250-27c) show that the basin was more environmentally diverse than a simple quartz beach.
BH250-88 and BH250-88b are best classified as talc-bearing phyllite, a fine-grained, low-grade metamorphic rock with strong foliation. In hand sample, the rock is gray to greenish-gray and has the soft, soapy feel typical of talc-rich rocks. Thin sections show abundant talc with high birefringence under crossed polars, along with crenulation cleavage that records multiple deformation phases. Minor minerals include Fe-Ti oxides, magnetite, and zircon, and the preservation of detrital zircon links the rock to its original sedimentary source.
The abundance of talc suggests a protolith very different from the surrounding quartz sandstone, most likely an impure dolostone, magnesium-rich carbonate mud, marl, or Mg-rich shale. While most of the Baraboo shoreline was dominated by quartz sand deposition, localized lagoons, tidal flats, or protected embayments likely allowed carbonate mud to accumulate.
During burial and regional metamorphism associated with the Penokean Orogeny, these Mg-rich sediments reacted with silica from surrounding quartz layers and metamorphic fluids, forming talc through decarbonation reactions:
Dolomite + quartz + water → talc + calcite + carbon dioxide
3CaMg(CO3)2 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Magnesite + quartz + water → talc + carbon dioxide
3MgCO3 + 4SiO2 + H2O → Mg3Si4O10(OH)2 +3CO2
In some cases, dolomite and quartz may have initially produced tremolite, which later altered to talc during retrograde metamorphism:
Dolomite + quartz + water → tremolite + calcite + carbon dioxide
5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
These reactions explain why talc occurs as a localized lithology within a dominantly quartz-rich succession. BH250-88 records a brief environmental shift in which carbonate-rich sediments accumulated within an otherwise mature shoreline before later metamorphism transformed them into talc-bearing phyllite, preserving an unexpected chapter in the history of the Baraboo Quartzite.
Minor Minerals: Fe-Ti oxides, magnetite, zircon
BH250-88 represents one of the most unusual lithologies preserved within the otherwise quartz-dominated Baraboo Quartzite succession. Deposited approximately 1.7 billion years ago, the Baraboo Quartzite primarily formed in a high-energy shoreline system where repeated wave action and sediment reworking produced exceptionally mature quartz sand. Rare lithologies such as talc-bearing phyllite and quartz-muscovite phyllite (e.g., BH250-27c) show that the basin was more environmentally diverse than a simple quartz beach.
BH250-88 and BH250-88b are best classified as talc-bearing phyllite, a fine-grained, low-grade metamorphic rock with strong foliation. In hand sample, the rock is gray to greenish-gray and has the soft, soapy feel typical of talc-rich rocks. Thin sections show abundant talc with high birefringence under crossed polars, along with crenulation cleavage that records multiple deformation phases. Minor minerals include Fe-Ti oxides, magnetite, and zircon, and the preservation of detrital zircon links the rock to its original sedimentary source.
The abundance of talc suggests a protolith very different from the surrounding quartz sandstone, most likely an impure dolostone, magnesium-rich carbonate mud, marl, or Mg-rich shale. While most of the Baraboo shoreline was dominated by quartz sand deposition, localized lagoons, tidal flats, or protected embayments likely allowed carbonate mud to accumulate.
During burial and regional metamorphism associated with the Penokean Orogeny, these Mg-rich sediments reacted with silica from surrounding quartz layers and metamorphic fluids, forming talc through decarbonation reactions:
Dolomite + quartz + water → talc + calcite + carbon dioxide
3CaMg(CO3)2 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Magnesite + quartz + water → talc + carbon dioxide
3MgCO3 + 4SiO2 + H2O → Mg3Si4O10(OH)2 +3CO2
In some cases, dolomite and quartz may have initially produced tremolite, which later altered to talc during retrograde metamorphism:
Dolomite + quartz + water → tremolite + calcite + carbon dioxide
5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
These reactions explain why talc occurs as a localized lithology within a dominantly quartz-rich succession. BH250-88 records a brief environmental shift in which carbonate-rich sediments accumulated within an otherwise mature shoreline before later metamorphism transformed them into talc-bearing phyllite, preserving an unexpected chapter in the history of the Baraboo Quartzite.
Coverage
Location: Baraboo, Wisconsin, USA
La Rue Quarry
GPS Location: 43°30'28.08"N, 89°45'28.11"W
La Rue Quarry
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 August 30, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/101.