BH250-39
Title
BH250-39
Subject
C-axis aragonite
Description
Major Mineral: aragonite
Mineralogy Comments: aragonite is cut perpendicular to the c-axis
BH250-39 is a thin section of aragonite, cut perpendicular to the c-axis. In this section, some grains appear optically isotropic, making them excellent candidates for displaying a biaxial interference figure (BXA(x)) with a small 2V angle. Aragonite has a 2V angle of approximately 18°, providing a clear example of what a small 2V looks like.
When rotating the stage 360°, the melatopes remain within the field of view, which is characteristic of minerals with a small 2V. Due to aragonite’s high birefringence, you will also observe a few isochromes surrounding the melatopes.
Instructional videos demonstrate the movement of the isogyres, melatopes, and isochromes, as well as the effect of inserting a mica plate. This plate helps reveal the locations of addition (blue) and subtraction (yellow) interference colors. The yellow (subtraction) colors appear on the convex side of the isogyre, while the blue (addition) colors appear on the concave side.
To determine the optic sign, recall the method of connecting the yellow (subtraction) areas with an imaginary line and extending it toward the inserted mica plate. If the line forms a minus (–) sign, the mineral is optically negative, confirming that aragonite is optically negative.
Mineralogy Comments: aragonite is cut perpendicular to the c-axis
BH250-39 is a thin section of aragonite, cut perpendicular to the c-axis. In this section, some grains appear optically isotropic, making them excellent candidates for displaying a biaxial interference figure (BXA(x)) with a small 2V angle. Aragonite has a 2V angle of approximately 18°, providing a clear example of what a small 2V looks like.
When rotating the stage 360°, the melatopes remain within the field of view, which is characteristic of minerals with a small 2V. Due to aragonite’s high birefringence, you will also observe a few isochromes surrounding the melatopes.
Instructional videos demonstrate the movement of the isogyres, melatopes, and isochromes, as well as the effect of inserting a mica plate. This plate helps reveal the locations of addition (blue) and subtraction (yellow) interference colors. The yellow (subtraction) colors appear on the convex side of the isogyre, while the blue (addition) colors appear on the concave side.
To determine the optic sign, recall the method of connecting the yellow (subtraction) areas with an imaginary line and extending it toward the inserted mica plate. If the line forms a minus (–) sign, the mineral is optically negative, confirming that aragonite is optically negative.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-39. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-39,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/44.