BH250-103c (Utah #88-A9)
Title
BH250-103c (Utah #88-A9)
Subject
Chondrodite and ludwigite Bearing Marble
Description
BH250-103C (Utah #88-A9)
Major Minerals: Chondrodite (humite group), ludwigite, calcite, dolomite
Sample BH250-103C (Utah #88-A9) was collected from the forsterite zone of the contact metamorphic aureole of the Alta Stock in Utah, but from a different locality than BH250-103 and BH250-103b. This sample is mineralogically significant for its abundance of coarse carbonate minerals, the presence of ludwigite, and smaller, nearly equigranular grains interpreted as chondrodite (of the humite group) rather than forsterite, which is more typical in other samples from the forsterite zone.
The carbonate minerals, primarily calcite/dolomite, occur as recrystallized grains that include large, well-developed twinned crystals. The very coarse-grained carbonates likely formed during peak contact metamorphism.
Notably, the calcite/dolomite crystals in BH250-103C (Utah #88-A9) are particularly well-suited for observing twin lamellae. This makes the sample valuable for optical mineralogy and petrographic studies, especially for:
Differentiating between calcite and dolomite using Miller index geometry,
Exploiting refractive index variations,
Accurately locating the c-axis in carbonate crystals to determine whether you are looking at calcite or dolomite.
In this sample, ludwigite is relatively common. Ludwigite is a boron-bearing magnesium-iron oxide mineral with the formula Mg₂FeBO₅, typically forming in contact metamorphosed, magnesium-rich carbonate rocks, particularly in magnesian skarns. It develops under high-temperature contact metamorphic conditions near igneous intrusions such as granite, granodiorite, or diorite stocks, as exemplified by the Alta Stock in Utah. The presence of ludwigite in this metamorphic aureole reflects boron enrichment introduced by metasomatic fluids, providing valuable insights into the fluid composition and metamorphic environment that governed skarn formation.
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
Major Minerals: Chondrodite (humite group), ludwigite, calcite, dolomite
Sample BH250-103C (Utah #88-A9) was collected from the forsterite zone of the contact metamorphic aureole of the Alta Stock in Utah, but from a different locality than BH250-103 and BH250-103b. This sample is mineralogically significant for its abundance of coarse carbonate minerals, the presence of ludwigite, and smaller, nearly equigranular grains interpreted as chondrodite (of the humite group) rather than forsterite, which is more typical in other samples from the forsterite zone.
The carbonate minerals, primarily calcite/dolomite, occur as recrystallized grains that include large, well-developed twinned crystals. The very coarse-grained carbonates likely formed during peak contact metamorphism.
Notably, the calcite/dolomite crystals in BH250-103C (Utah #88-A9) are particularly well-suited for observing twin lamellae. This makes the sample valuable for optical mineralogy and petrographic studies, especially for:
Differentiating between calcite and dolomite using Miller index geometry,
Exploiting refractive index variations,
Accurately locating the c-axis in carbonate crystals to determine whether you are looking at calcite or dolomite.
In this sample, ludwigite is relatively common. Ludwigite is a boron-bearing magnesium-iron oxide mineral with the formula Mg₂FeBO₅, typically forming in contact metamorphosed, magnesium-rich carbonate rocks, particularly in magnesian skarns. It develops under high-temperature contact metamorphic conditions near igneous intrusions such as granite, granodiorite, or diorite stocks, as exemplified by the Alta Stock in Utah. The presence of ludwigite in this metamorphic aureole reflects boron enrichment introduced by metasomatic fluids, providing valuable insights into the fluid composition and metamorphic environment that governed skarn formation.
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 103c. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-103c (Utah #88-A9),” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/121.