BH250-103b

BH250-103b

Centered Opic Axis Figure

BH250-103b (Marble, Alta, Utah) – Calcite interference figures

Calcite in BH250-103b provides a realistic and instructive setting for learning interference figures because students must find the right grain rather than being given a perfect orientation. Begin in PPL and XPL by scanning the mosaic of recrystallized calcite for grains that are clean, inclusion-poor, and as equant as possible. Unlike ideal teaching sections, most grains will not be perfectly oriented. The goal is to identify grains that show very low birefringence at their center in XPL, often appearing nearly dark or with subdued first-order colors—an indication that you are looking close to the optic axis (c-axis) of this uniaxial mineral.

Once a promising grain is located, center it carefully and switch to conoscopic observation. With proper alignment, calcite will produce a uniaxial optic axis figure, characterized by a centered melatope and a dark cross (isogyres) that remains centered during stage rotation. Because calcite has high birefringence, the isochromes are vivid and well developed, making the figure easy to interpret once the correct grain is found. Small rotations of the stage should keep the cross stable and centered, confirming that you are truly looking down the optic axis. In addition, BH250-103b contains grains that are slightly off the ideal orientation, producing off-centered optic axis figures where the melatope remains within the field of view but is displaced from the center. In these cases, part of the isogyre will sweep toward or out of the field during stage rotation—an important intermediate condition that helps students understand how figures evolve with orientation.

To complete the analysis, insert the mica (sensitive tint) plate. Calcite is optically negative, so you should observe characteristic color changes in opposite quadrants of the figure that correspond to the fast and slow vibration directions. You can also use a quartz wedge test, where additional colors move toward the melatope in the addition quadrantsand move away from it in the subtraction quadrants, reinforcing the interpretation of vibration directions and optic sign. The key teaching value of BH250-103b is that students must work through the process—scanning, rejecting poorly oriented grains, and refining their choice—before obtaining a clean interference figure. This builds the skill of recognizing the right grain in a natural rock, which is far more transferable than working with idealized samples.

BH250-36 (Single calcite crystal, c ⟂ section) – Reference standard

Calcite in BH250-36 represents an idealized case: a single crystal cut perpendicular to the c-axis, meaning you are already looking directly down the optic axis. In XPL, the grain will show very low birefringence at the center, and in conoscopic view it produces a perfect uniaxial optic axis figure with a sharply centered cross and concentric isochromes. The figure remains fixed during stage rotation, providing a textbook example of a uniaxial interference figure.

While BH250-36 is useful as a reference for what a “perfect” figure looks like, it removes the most important step in real petrographic work—finding the correct grain. For that reason, students should rely on BH250-103b to develop their observational skills and use BH250-36 primarily as a visual benchmark for comparison and confirmation.