BH250-31
Title
BH250-31
Subject
BXa = Biaxial optic axis figure
Description
BH250-31 offers an excellent opportunity to learn how to intentionally locate and use the “right” grain for interference figure work. In this talc–tremolite schist, begin in plane-polarized light (PPL) and crossed polars (XPL) by scanning for talc grains that appear clean, inclusion-free, and relatively equant in shape. The most useful grains are those cut in an orientation that allows you to look down the X direction, producing a Bxa (acute bisectrix) figure, where X = α = the fast vibration direction. In XPL, these grains will show very low birefringence at their center, often appearing nearly dark or displaying only very subdued interference colors. That is your first clue that you are looking down, or very close to, the appropriate optical direction and that you have found a grain capable of producing a high-quality interference figure.
Once a suitable grain is identified, center it precisely in the field of view and switch to conoscopic observation, either by inserting the Bertrand lens or by removing the eyepiece. Carefully adjust the focus and illumination to bring out the interference figure. In BH250-31, well-oriented talc grains produce a clear Bxa acute bisectrix figure, meaning you are looking down the X direction (α, the fast vibration direction). The figure is characterized by a centered melatope and symmetrical isogyres. Because talc has low birefringence, the isochromes are subtle, but the geometry of the figure remains well defined. Small stage rotations should keep the melatope near the center, confirming that the grain is properly aligned for this interference figure.
To refine the interpretation further, insert the mica (sensitive tint) plate. In this sample, the plate reveals a negative optic sign and a small 2V angle of about 12°. As the plate is inserted, observe the color changes in the quadrants, or in the concave and convex parts of the isogyres, and relate these changes to the slow and fast vibration directions. The combination of a centered Bxa figure, low birefringence at the grain center, and the response to the mica plate provides a complete step-by-step pathway from grain selection to optical interpretation, making BH250-31 a particularly effective teaching specimen for mastering interference figures.
Once a suitable grain is identified, center it precisely in the field of view and switch to conoscopic observation, either by inserting the Bertrand lens or by removing the eyepiece. Carefully adjust the focus and illumination to bring out the interference figure. In BH250-31, well-oriented talc grains produce a clear Bxa acute bisectrix figure, meaning you are looking down the X direction (α, the fast vibration direction). The figure is characterized by a centered melatope and symmetrical isogyres. Because talc has low birefringence, the isochromes are subtle, but the geometry of the figure remains well defined. Small stage rotations should keep the melatope near the center, confirming that the grain is properly aligned for this interference figure.
To refine the interpretation further, insert the mica (sensitive tint) plate. In this sample, the plate reveals a negative optic sign and a small 2V angle of about 12°. As the plate is inserted, observe the color changes in the quadrants, or in the concave and convex parts of the isogyres, and relate these changes to the slow and fast vibration directions. The combination of a centered Bxa figure, low birefringence at the grain center, and the response to the mica plate provides a complete step-by-step pathway from grain selection to optical interpretation, making BH250-31 a particularly effective teaching specimen for mastering interference figures.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-31. Housed at Carleton College in Minnesota
Publisher
Bereket Haileab
Contributor
Bereket Haileab
Relation
Collection
Citation
Bereket Haileab, “BH250-31,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/375.