BH250-36
Title
BH250-36
Subject
C-axis Calcite
Description
Major Mineral: Calcite
Mineralogy Comments: Calcite grain cut perpendicular to the c-axis.
Optics: A centered optic axis figure displays a black cross (isogyres) with a central point—the melatope—indicating the emergence of the optic axis. Surrounding the melatope are concentric rings of interference colors known as isochromes. Due to calcite’s high birefringence (~0.172), multiple isochromes are visible and well defined. Calcite is optically uniaxial negative.
When viewed with a sensitive tint plate (mica), addition (yellow) appears in quadrants II and IV, and subtraction (blue) in quadrants I and III, consistent with a uniaxial negative mineral. However, using a quartz wedge reverses these interference color changes: subtraction occurs in quadrants II and IV, and addition in quadrants I and III.
The BH250 collection includes a variety of rocks such as marbles, skarns, and amygdaloidal basalts with secondary minerals filling cavities, many containing calcite. In these samples, calcite grains are often oriented perpendicular to the c-axis, facilitating clear observation of interference figures. Because of calcite’s high birefringence, sections cut perpendicular to the c-axis do not appear isotropic and never go completely dark under crossed polars. Off-centered optic axis figures are also commonly observed, where the melatope is displaced from the center and portions of the isogyre sweep in and out of the field of view during stage rotation.
Comparison: Compare the optics observed in BH250-36 with those in BH250-103b (marble). In BH250-36, the orientation is ideal and the interference figure is perfectly centered and easy to interpret. In contrast, BH250-103b requires careful grain selection; many grains will show slightly off-centered optic axis figures, partial isogyres, or only near-optic-axis conditions. This comparison highlights the difference between a textbook example and the more realistic challenge of identifying and interpreting interference figures in natural rocks.
Mineralogy Comments: Calcite grain cut perpendicular to the c-axis.
Optics: A centered optic axis figure displays a black cross (isogyres) with a central point—the melatope—indicating the emergence of the optic axis. Surrounding the melatope are concentric rings of interference colors known as isochromes. Due to calcite’s high birefringence (~0.172), multiple isochromes are visible and well defined. Calcite is optically uniaxial negative.
When viewed with a sensitive tint plate (mica), addition (yellow) appears in quadrants II and IV, and subtraction (blue) in quadrants I and III, consistent with a uniaxial negative mineral. However, using a quartz wedge reverses these interference color changes: subtraction occurs in quadrants II and IV, and addition in quadrants I and III.
The BH250 collection includes a variety of rocks such as marbles, skarns, and amygdaloidal basalts with secondary minerals filling cavities, many containing calcite. In these samples, calcite grains are often oriented perpendicular to the c-axis, facilitating clear observation of interference figures. Because of calcite’s high birefringence, sections cut perpendicular to the c-axis do not appear isotropic and never go completely dark under crossed polars. Off-centered optic axis figures are also commonly observed, where the melatope is displaced from the center and portions of the isogyre sweep in and out of the field of view during stage rotation.
Comparison: Compare the optics observed in BH250-36 with those in BH250-103b (marble). In BH250-36, the orientation is ideal and the interference figure is perfectly centered and easy to interpret. In contrast, BH250-103b requires careful grain selection; many grains will show slightly off-centered optic axis figures, partial isogyres, or only near-optic-axis conditions. This comparison highlights the difference between a textbook example and the more realistic challenge of identifying and interpreting interference figures in natural rocks.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-36. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-36,” BH250 Mineralogy Teaching Collection, accessed April 26, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/40.