BH250-105
Title
BH250-105
Subject
Talc Bearing Marble
Description
Major Mineral: talc, calcite, dolomite
Rock and Thin Section Description:
This sample is from the outermost metamorphic zone of the Alta Stock aureole, approximately 4 km from the granodiorite contact. The talc and associated carbonate minerals are very fine-grained, making them challenging to identify without careful observation. In thin section, talc is present but requires close examination due to its subtle appearance and fine grain size. This zone corresponds to the talc zone, which marks the lowest-temperature conditions in the aureole's metamorphic gradient. It formed through the thermal metamorphism of Mg-rich, carbonate-bearing protoliths, particularly dolomitic marbles, under low-temperature, high-CO₂ conditions along the outer margin of the aureole.
Occurrence and Formation: Talc typically forms via the reaction of dolomite and quartz in the presence of water, producing talc and releasing CO₂:
3Mg3CA(CO3)2 + 4SiO2 + H2O = 3CaCO3 + 3Mg3SiO4(OH)2 + 3CO2
Talc in the Alta Stock aureole represents the initial stage of thermal metamorphism, prior to the stabilization of higher-temperature minerals like tremolite, diopside, forsterite and periclase. Its occurrence is important for understanding the thermal gradient and fluid interactions across the metamorphic aureole and serves as a key index for mapping the extent of metamorphic influence from the intrusion.
Rock and Thin Section Description:
This sample is from the outermost metamorphic zone of the Alta Stock aureole, approximately 4 km from the granodiorite contact. The talc and associated carbonate minerals are very fine-grained, making them challenging to identify without careful observation. In thin section, talc is present but requires close examination due to its subtle appearance and fine grain size. This zone corresponds to the talc zone, which marks the lowest-temperature conditions in the aureole's metamorphic gradient. It formed through the thermal metamorphism of Mg-rich, carbonate-bearing protoliths, particularly dolomitic marbles, under low-temperature, high-CO₂ conditions along the outer margin of the aureole.
Occurrence and Formation: Talc typically forms via the reaction of dolomite and quartz in the presence of water, producing talc and releasing CO₂:
3Mg3CA(CO3)2 + 4SiO2 + H2O = 3CaCO3 + 3Mg3SiO4(OH)2 + 3CO2
Talc in the Alta Stock aureole represents the initial stage of thermal metamorphism, prior to the stabilization of higher-temperature minerals like tremolite, diopside, forsterite and periclase. Its occurrence is important for understanding the thermal gradient and fluid interactions across the metamorphic aureole and serves as a key index for mapping the extent of metamorphic influence from the intrusion.
Coverage
Location: Alta, Utah, USA
GPS Coordinates: 40°34'35.81"N, 111°36'31.98"W
GPS Coordinates: 40°34'35.81"N, 111°36'31.98"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-105. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-105,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/124.