BH250-103b (Utah #88-C5)
Title
BH250-103b (Utah #88-C5)
Subject
Forsterite Bearing Marble
Description
Major Minerals: Forsterite, periclase, calcite, dolomite
Sample BH250-103b (Utah #88-C5) was collected from the forsterite zone of the contact metamorphic aureole surrounding the Alta Stock in Utah. This sample is mineralogically significant due to its abundance of coarser, unaltered, and notably elongate forsterite crystals. In hand sample, these forsterite grains stand out as prominent, white, and prismatic crystals that exhibit excellent preservation despite the high-temperature conditions of formation.
Under the petrographic microscope, sections of forsterite cut perpendicular to the optic axis yield a straight-line isogyre in the interference figure, consistent with its biaxial optical character and a measured 2V angle of approximately 90 degrees. This diagnostic feature further confirms the identity and quality of the forsterite (100 % Mg endmember) in this sample.
Associated carbonate minerals, predominantly calcite and dolomite, are present as large, recrystallized grains. Many of these grains are oriented such that they are nearly perpendicular to their c-axis, making them ideal for observing centered optic axis, off centered optic axis and really off centered optic axis interference figures in thin section. There are five videos that show these interference figures that you will find when you scroll down the page. This provides excellent teaching and reference material for demonstrating carbonate optical properties in mineralogy courses.
When examining a mineral with high birefringence, such as calcite, the interference figure displays an isogyre surrounded by multiple isochromes centered around the melatope. By inserting the mica plate (also called a gypsum plate or accessory plate), colors in the interference figure will add or subtract in opposite quadrants.
To determine the optic sign, look for the yellow color indicating subtraction. Connect the two quadrants showing yellow, extend an imaginary line between them, and align it with the known orientation of the mica plate’s fast ray. If this line forms a plus sign (+) with the fast ray of the plate, the mineral is optically positive. If it forms a minus sign (-), the mineral is optically negative.
Alternatively, when using a quartz wedge on a microscope equipped for it, slowly insert the wedge while observing the interference figure. If the colors appear to move outward from the center of the figure (colors "flying out"), this indicates subtraction, meaning the mineral is optically negative. If the colors move inward toward the center ("flying in"), this indicates addition, meaning the mineral is optically positive.
Compare the calcites in this thin sections to those in BH250-36.
Periclase (un altered), another key phase in this sample, occurs as isotropic grains, though its presence is more limited compared to the silicates and carbonates.
Overall, BH250-103b (Utah #88-C5) exemplifies the mineralogical and textural characteristics typical of high-temperature contact metamorphic marbles in the forsterite zone, preserving both the primary metamorphic assemblage and optical qualities that make it suitable for both research and educational purposes. These samples of BH250-102 to BH250-105 are definitely some of the best examples of contact metamorphism. Glad the Alta Stock area is not developed by developers and will stay that way so that future geologist will continue to visit and learn from this amazing place in the world.
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 BH250-103b (Utah #88-C5) was collected from the forsterite zone of the contact metamorphic aureole surrounding the Alta Stock in Utah. This sample is mineralogically significant due to its abundance of coarser, unaltered, and notably elongate forsterite crystals. In hand sample, these forsterite grains stand out as prominent, white, and prismatic crystals that exhibit excellent preservation despite the high-temperature conditions of formation.
Under the petrographic microscope, sections of forsterite cut perpendicular to the optic axis yield a straight-line isogyre in the interference figure, consistent with its biaxial optical character and a measured 2V angle of approximately 90 degrees. This diagnostic feature further confirms the identity and quality of the forsterite (100 % Mg endmember) in this sample.
Associated carbonate minerals, predominantly calcite and dolomite, are present as large, recrystallized grains. Many of these grains are oriented such that they are nearly perpendicular to their c-axis, making them ideal for observing centered optic axis, off centered optic axis and really off centered optic axis interference figures in thin section. There are five videos that show these interference figures that you will find when you scroll down the page. This provides excellent teaching and reference material for demonstrating carbonate optical properties in mineralogy courses.
When examining a mineral with high birefringence, such as calcite, the interference figure displays an isogyre surrounded by multiple isochromes centered around the melatope. By inserting the mica plate (also called a gypsum plate or accessory plate), colors in the interference figure will add or subtract in opposite quadrants.
To determine the optic sign, look for the yellow color indicating subtraction. Connect the two quadrants showing yellow, extend an imaginary line between them, and align it with the known orientation of the mica plate’s fast ray. If this line forms a plus sign (+) with the fast ray of the plate, the mineral is optically positive. If it forms a minus sign (-), the mineral is optically negative.
Alternatively, when using a quartz wedge on a microscope equipped for it, slowly insert the wedge while observing the interference figure. If the colors appear to move outward from the center of the figure (colors "flying out"), this indicates subtraction, meaning the mineral is optically negative. If the colors move inward toward the center ("flying in"), this indicates addition, meaning the mineral is optically positive.
Compare the calcites in this thin sections to those in BH250-36.
Periclase (un altered), another key phase in this sample, occurs as isotropic grains, though its presence is more limited compared to the silicates and carbonates.
Overall, BH250-103b (Utah #88-C5) exemplifies the mineralogical and textural characteristics typical of high-temperature contact metamorphic marbles in the forsterite zone, preserving both the primary metamorphic assemblage and optical qualities that make it suitable for both research and educational purposes. These samples of BH250-102 to BH250-105 are definitely some of the best examples of contact metamorphism. Glad the Alta Stock area is not developed by developers and will stay that way so that future geologist will continue to visit and learn from this amazing place in the world.
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 BH250-103b. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-103b (Utah #88-C5),” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/120.