BH250-102b (original Utah number 88-A5)
Title
BH250-102b (original Utah number 88-A5)
Subject
Marble
Description
Major Minerals: Periclase, forsterite (olivine), brucite, serpentine, calcite
Alteration: Periclase alters to brucite; olivine alters to serpentine.
Optical Properties:
Periclase is a high-relief, isotropic mineral. In thin section, it is easily recognized by its high relief (n = 1.736), isotropic character (remaining dark in all orientations under crossed polars), and equant grain shapes.
Periclase is susceptible to alteration during water-rock interaction, leading to the formation of brucite. In thin section, brucite is identified by its moderate birefringence, low relief, and fibrous habit. Altered periclase grains are often preserved as cores within brucite crystals.
There are small amounts of forsterite associated with periclase in the periclase zone. The occurrence of forsterite in this zone of the Alta Stock metamorphic aureole reflects the local bulk composition of the protolith and specific metamorphic reactions that occur under the high-temperature, low- to moderate-pressure conditions typical of contact metamorphism.
The periclase zone develops in Mg-rich, low-silica carbonate rocks (such as dolomitic marble) when subjected to high temperatures. Forsterite (Mg₂SiO₄) forms in this zone when small amounts of silica are present in the system, enabling the reaction:
Dolomite + Quartz → Forsterite + Calcite + CO₂
3CaMg(CO₃)₂ + SiO₂ → Mg₂SiO₄ + 3CaCO₃ + 3CO₂
This reaction requires silica (usually as quartz or introduced through fluid interaction, as siliceous dolomite), dolomite, and sufficient temperature.
Texture:
Recrystallized calcite and dolomite are significantly larger in grain size compared to the original unmetamorphosed siliceous dolostone.
Rock Outcrop:
This rock sample was collected adjacent to the Alta Stock granodiorite (BH250-101). The original rock was a siliceous dolomite. Contact metamorphism caused by the intrusion of the Alta Stock granodiorite metamorphosed the dolomite, recrystallizing it into a coarse-grained marble composed of calcite and dolomite. BH250-102, 102b and 102c are similar with the exception of 102c having larger forsterite (olivine) crystals. The outcrop is a popular destination for field trips and geology courses.
Metamorphic Reactions:
The likely metamorphic reactions leading to the formation of periclase and brucite are:
Dolomite breakdown:
Dolomite → Periclase + Calcite + Carbon dioxide
CaMg(CO₃)₂ → MgO + CaCO₃ + CO₂
Hydration of periclase:
Periclase + Water → Brucite
MgO + H₂O → Mg(OH)₂
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
Alteration: Periclase alters to brucite; olivine alters to serpentine.
Optical Properties:
Periclase is a high-relief, isotropic mineral. In thin section, it is easily recognized by its high relief (n = 1.736), isotropic character (remaining dark in all orientations under crossed polars), and equant grain shapes.
Periclase is susceptible to alteration during water-rock interaction, leading to the formation of brucite. In thin section, brucite is identified by its moderate birefringence, low relief, and fibrous habit. Altered periclase grains are often preserved as cores within brucite crystals.
There are small amounts of forsterite associated with periclase in the periclase zone. The occurrence of forsterite in this zone of the Alta Stock metamorphic aureole reflects the local bulk composition of the protolith and specific metamorphic reactions that occur under the high-temperature, low- to moderate-pressure conditions typical of contact metamorphism.
The periclase zone develops in Mg-rich, low-silica carbonate rocks (such as dolomitic marble) when subjected to high temperatures. Forsterite (Mg₂SiO₄) forms in this zone when small amounts of silica are present in the system, enabling the reaction:
Dolomite + Quartz → Forsterite + Calcite + CO₂
3CaMg(CO₃)₂ + SiO₂ → Mg₂SiO₄ + 3CaCO₃ + 3CO₂
This reaction requires silica (usually as quartz or introduced through fluid interaction, as siliceous dolomite), dolomite, and sufficient temperature.
Texture:
Recrystallized calcite and dolomite are significantly larger in grain size compared to the original unmetamorphosed siliceous dolostone.
Rock Outcrop:
This rock sample was collected adjacent to the Alta Stock granodiorite (BH250-101). The original rock was a siliceous dolomite. Contact metamorphism caused by the intrusion of the Alta Stock granodiorite metamorphosed the dolomite, recrystallizing it into a coarse-grained marble composed of calcite and dolomite. BH250-102, 102b and 102c are similar with the exception of 102c having larger forsterite (olivine) crystals. The outcrop is a popular destination for field trips and geology courses.
Metamorphic Reactions:
The likely metamorphic reactions leading to the formation of periclase and brucite are:
Dolomite breakdown:
Dolomite → Periclase + Calcite + Carbon dioxide
CaMg(CO₃)₂ → MgO + CaCO₃ + CO₂
Hydration of periclase:
Periclase + Water → Brucite
MgO + H₂O → Mg(OH)₂
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
GPS Coordinates: 40°34'54.57"N, 111°36'39.25"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 102b. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-102b (original Utah number 88-A5),” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/117.