BH250-104
Title
BH250-104
Subject
Tremolite Bearing Marble
Description
Major Mineral: Tremolite, calcite, dolomite
In the contact metamorphic aureole of the Alta Stock, Utah, tremolite occurs within the tremolite zone, a distinct zone formed in carbonate-rich rocks, especially dolomitic marbles, due to the influence of the granodioritic intrusion. Tremolite develops in Mg- and Ca-rich dolomitic protoliths, typically alongside calcite, dolomite, diopside, and occasionally forsterite, depending on temperature and fluid availability. Its presence generally indicates lower to intermediate temperatures in the aureole, forming between the talc zone and the forsterite zone, before higher-temperature minerals like forsterite appear closer to the intrusion.
Tremolite forms through the decarbonation of dolomite and quartz, producing CO₂ a key reaction in the contact metamorphism of carbonates. In thin section, tremolite is colorless to pale green, commonly prismatic or fibrous in habit. The metamorphic progression around the Alta Stock is typically: Talc Zone (talc + dolomite), Tremolite Zone (tremolite + calcite/dolomite), Forsterite Zone (forsterite + calcite + diopside), and Periclase Zone (periclase + calcite/dolomite).
5CaMg(CO3)2 + 8SiO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
Thus, tremolite serves as an important index mineral, marking the intermediate metamorphic grade and providing valuable insights into the metamorphic conditions and fluid evolution within the Alta Stock aureole.
Within the tremolite-bearing zone, large calcite and dolomite crystals are present alongside tremolite. These carbonate minerals commonly exhibit well-developed twin lamellae and often display anomalous interference colors, rather than the typical high birefringence pastel colors expected of carbonates. This optical behavior makes them excellent candidates for obtaining centered optic axis interference figures in thin section studies.
Rocks and Thin sections of BH250-104: Some thin sections show plenty of tremolite through the thin sections, however, older thin sections, you have to look hard to find the tremolite.
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
In summary, tremolite in the Alta Stock contact aureole is an important index mineral, representing an intermediate metamorphic grade in the progression from talc- to forsterite-bearing marbles, providing insight into metamorphic conditions and fluid interactions in the aureole.
In the contact metamorphic aureole of the Alta Stock, Utah, tremolite occurs within the tremolite zone, a distinct zone formed in carbonate-rich rocks, especially dolomitic marbles, due to the influence of the granodioritic intrusion. Tremolite develops in Mg- and Ca-rich dolomitic protoliths, typically alongside calcite, dolomite, diopside, and occasionally forsterite, depending on temperature and fluid availability. Its presence generally indicates lower to intermediate temperatures in the aureole, forming between the talc zone and the forsterite zone, before higher-temperature minerals like forsterite appear closer to the intrusion.
Tremolite forms through the decarbonation of dolomite and quartz, producing CO₂ a key reaction in the contact metamorphism of carbonates. In thin section, tremolite is colorless to pale green, commonly prismatic or fibrous in habit. The metamorphic progression around the Alta Stock is typically: Talc Zone (talc + dolomite), Tremolite Zone (tremolite + calcite/dolomite), Forsterite Zone (forsterite + calcite + diopside), and Periclase Zone (periclase + calcite/dolomite).
5CaMg(CO3)2 + 8SiO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
Thus, tremolite serves as an important index mineral, marking the intermediate metamorphic grade and providing valuable insights into the metamorphic conditions and fluid evolution within the Alta Stock aureole.
Within the tremolite-bearing zone, large calcite and dolomite crystals are present alongside tremolite. These carbonate minerals commonly exhibit well-developed twin lamellae and often display anomalous interference colors, rather than the typical high birefringence pastel colors expected of carbonates. This optical behavior makes them excellent candidates for obtaining centered optic axis interference figures in thin section studies.
Rocks and Thin sections of BH250-104: Some thin sections show plenty of tremolite through the thin sections, however, older thin sections, you have to look hard to find the tremolite.
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
In summary, tremolite in the Alta Stock contact aureole is an important index mineral, representing an intermediate metamorphic grade in the progression from talc- to forsterite-bearing marbles, providing insight into metamorphic conditions and fluid interactions in the aureole.
Coverage
Location: Alta, Utah, USA
GPS Coordinates: 40°34'48.68"N, 111°36'34.67"W
GPS Coordinates: 40°34'48.68"N, 111°36'34.67"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 104. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-104,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/123.