BH250-103
Title
BH250-103
Subject
Forsterite Bearing Marble
Description
Major Minerals: Long prismatic altered forsterite, periclase, calcite, dolomite
Sample BH250-103 is collected from the forsterite zone within the contact metamorphic aureole of the Alta Stock in Utah. This sample is mineralogically significant due to the presence of very long prismatic crystals of forsterite, which are now completely altered to serpentine. The pseudomorphic replacement preserves the original crystal habit of forsterite, making the alteration texture visually striking under the microscope.
Additionally, periclase (MgO) is present in this sample as unaltered grains, along with some partially altered grains, especially when compared to the better-preserved periclase in the periclase zone of sample BH250-102. The alteration of periclase typically progresses to brucite (Mg(OH)₂) under retrograde metamorphic conditions. However, further petrographic or geochemical analysis would be required to confirm the specific alteration product present in this sample.
The carbonate minerals, calcite and dolomite, occur as recrystallized grains displaying bimodal grain sizes: Very coarse-grained carbonates, which likely formed during the peak of contact metamorphism, Medium-grained carbonates, which may reflect recrystallization during retrograde metamorphism or subsequent hydrothermal fluid interactions.
The coexistence of forsterite (altered to serpentine), periclase, and recrystallized carbonates reflects high-temperature, magnesium-rich skarn assemblages characteristic of contact metamorphism in carbonate protoliths. The textural relationships and mineral assemblage provide insights into the prograde and retrograde metamorphic history of the Alta Stock aureole.
Summary of Alta Stock Metamorphic Progression (simplified):
Talc Zone: Talc + dolomite
Tremolite Zone: Tremolite + calcite/dolomite
Forsterite Zone: Forsterite + calcite + diopside
Periclase Zone: Periclase + calcite/dolomite
Sample BH250-103 is collected from the forsterite zone within the contact metamorphic aureole of the Alta Stock in Utah. This sample is mineralogically significant due to the presence of very long prismatic crystals of forsterite, which are now completely altered to serpentine. The pseudomorphic replacement preserves the original crystal habit of forsterite, making the alteration texture visually striking under the microscope.
Additionally, periclase (MgO) is present in this sample as unaltered grains, along with some partially altered grains, especially when compared to the better-preserved periclase in the periclase zone of sample BH250-102. The alteration of periclase typically progresses to brucite (Mg(OH)₂) under retrograde metamorphic conditions. However, further petrographic or geochemical analysis would be required to confirm the specific alteration product present in this sample.
The carbonate minerals, calcite and dolomite, occur as recrystallized grains displaying bimodal grain sizes: Very coarse-grained carbonates, which likely formed during the peak of contact metamorphism, Medium-grained carbonates, which may reflect recrystallization during retrograde metamorphism or subsequent hydrothermal fluid interactions.
The coexistence of forsterite (altered to serpentine), periclase, and recrystallized carbonates reflects high-temperature, magnesium-rich skarn assemblages characteristic of contact metamorphism in carbonate protoliths. The textural relationships and mineral assemblage provide insights into the prograde and retrograde metamorphic history of the Alta Stock aureole.
Summary of Alta Stock Metamorphic Progression (simplified):
Talc Zone: Talc + dolomite
Tremolite Zone: Tremolite + calcite/dolomite
Forsterite Zone: Forsterite + calcite + diopside
Periclase Zone: Periclase + calcite/dolomite
Coverage
Location: Alta, Utah, USA
GPS Coordinates: 40°34'54.57"N, 111°36'39.25"W
GPS Coordinates: 40°34'54.57"N, 111°36'39.25"W
Creator
Bereket Haileab
John Bowman
John Bowman
Source
From the rock collection of Bereket Haileab. Sample BH250-103. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab
John Bowman, “BH250-103,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/119.