BH250-73
Title
BH250-73
Subject
Talc-tremolite Schist
Description
Major Minerals: talc, tremolite, anorthite
Minor Mineral: chlorite
In this thin section, tremolite crystals are prominently featured, some of which are oriented perpendicular to the optic axis, providing excellent interference figures when viewed under crossed polars. These figures are characterized by highly curved isogyres, which are diagnostic for monoclinic amphiboles like tremolite. The tremolite displays a strong foliation, with crystals aligned parallel to the foliation plane, and exhibits the typical amphibole cleavage at approximately 56° and 124°, clearly visible in the section. The grains are consistently elongated and prismatic, contributing to the pronounced linear fabric observed in both plane-polarized and cross-polarized light.
In thin section, many tremolite grains are cut perpendicular to an optic-axis, showing perfect biaxial interference figures with isogyres that are almost straight line. These grains display a large 2V angle and are optically negative, as demonstrated in the videos attached to this page.
Talc in the same section is distinguished by its perfect basal cleavage, producing smooth, platy grains that align along the foliation. Talc shows high birefringence (see video), often displaying brilliant interference colors that can range from second to third order. These optical behaviors can sometimes lead to confusion with muscovite, especially among students who are taking optics for the first time, due to their similar appearance under crossed polars. However, talc can be differentiated by its lower 2V angle, its distinct pearly luster in hand sample, and importantly, its paragenetic association with amphiboles like tremolite in metamorphic rocks such as talc schists.
The combination of textural relationships, talc plates often intergrown with or wrapping around tremolite prisms, and optical properties, including cleavage, birefringence, and optic sign, provides reliable means for distinguishing between talc and muscovite in thin section.
Minor Mineral: chlorite
In this thin section, tremolite crystals are prominently featured, some of which are oriented perpendicular to the optic axis, providing excellent interference figures when viewed under crossed polars. These figures are characterized by highly curved isogyres, which are diagnostic for monoclinic amphiboles like tremolite. The tremolite displays a strong foliation, with crystals aligned parallel to the foliation plane, and exhibits the typical amphibole cleavage at approximately 56° and 124°, clearly visible in the section. The grains are consistently elongated and prismatic, contributing to the pronounced linear fabric observed in both plane-polarized and cross-polarized light.
In thin section, many tremolite grains are cut perpendicular to an optic-axis, showing perfect biaxial interference figures with isogyres that are almost straight line. These grains display a large 2V angle and are optically negative, as demonstrated in the videos attached to this page.
Talc in the same section is distinguished by its perfect basal cleavage, producing smooth, platy grains that align along the foliation. Talc shows high birefringence (see video), often displaying brilliant interference colors that can range from second to third order. These optical behaviors can sometimes lead to confusion with muscovite, especially among students who are taking optics for the first time, due to their similar appearance under crossed polars. However, talc can be differentiated by its lower 2V angle, its distinct pearly luster in hand sample, and importantly, its paragenetic association with amphiboles like tremolite in metamorphic rocks such as talc schists.
The combination of textural relationships, talc plates often intergrown with or wrapping around tremolite prisms, and optical properties, including cleavage, birefringence, and optic sign, provides reliable means for distinguishing between talc and muscovite in thin section.
Coverage
Location: St. Lawrence County, New York, USA
GPS Coordinates: 44°15'58.27"N, 75°24'18.82"W
GPS Coordinates: 44°15'58.27"N, 75°24'18.82"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-73. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-73,” BH250 Mineralogy Teaching Collection, accessed April 26, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/83.