BH250-102c
Title
BH250-102c
Subject
Periclase Bearing Marble
Description
Major Minerals: Periclase, forsterite (olivine), brucite, serpentine, calcite/dolomite
Alteration: Periclase alters to brucite; forsterite (olivine) is altered 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:
The metamorphic reaction that produces periclase is:
Dolomite → Periclase + Calcite + CO₂.
This means that in a sample that has undergone metamorphism, both calcite and dolomite may coexist. Since these two minerals have similar optical properties, distinguishing them requires careful observation. The following steps outline how to tell them apart:
Recrystallized calcite and dolomite exhibit significantly larger grain sizes compared to the original unmetamorphosed siliceous dolostone. Both minerals display twin lamellae, a common textural feature in calcite and dolomite.
In optical mineralogy, you can use the position of the relief change as you rotate the stage to identify the c-axis. Then, by using the Miller indices of the twin lamellae and examining the axial relationship between the c-axis and the twin lamellae, specifically whether the intersection forms an acute or obtuse angle, you can determine whether the carbonate mineral is calcite or dolomite.
Rock Outcrop:
This rock sample was collected adjacent to the granodiorite of Alta Stock (BH250-101). It shows strong alteration of periclase and olivine than BH250-102 and 102b. BH250-102, 102b and 102c are similar with the exception of 102c having larger forsterite (olivine) crystals. The outcrop is a common destination for field trips and geology field 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; forsterite (olivine) is altered 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:
The metamorphic reaction that produces periclase is:
Dolomite → Periclase + Calcite + CO₂.
This means that in a sample that has undergone metamorphism, both calcite and dolomite may coexist. Since these two minerals have similar optical properties, distinguishing them requires careful observation. The following steps outline how to tell them apart:
Recrystallized calcite and dolomite exhibit significantly larger grain sizes compared to the original unmetamorphosed siliceous dolostone. Both minerals display twin lamellae, a common textural feature in calcite and dolomite.
In optical mineralogy, you can use the position of the relief change as you rotate the stage to identify the c-axis. Then, by using the Miller indices of the twin lamellae and examining the axial relationship between the c-axis and the twin lamellae, specifically whether the intersection forms an acute or obtuse angle, you can determine whether the carbonate mineral is calcite or dolomite.
Rock Outcrop:
This rock sample was collected adjacent to the granodiorite of Alta Stock (BH250-101). It shows strong alteration of periclase and olivine than BH250-102 and 102b. BH250-102, 102b and 102c are similar with the exception of 102c having larger forsterite (olivine) crystals. The outcrop is a common destination for field trips and geology field 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 102c. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-102c,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/118.