BH250-41
Title
BH250-41
Subject
BH250-41 (Dunite) – Forsterite interference figures
Description
BH250-41 (Dunite) – Forsterite interference figures
BH250-41 is an excellent sample for learning the optical behavior of forsteritic olivine in thin section. In this dunite, several olivine grains are cut nearly perpendicular to an optic axis and produce a distinctive interference figure. Begin by scanning the thin section in plane-polarized light (PPL) and crossed polars (XPL) for grains that are clean, inclusion-poor, and relatively broad in view. In XPL, the best grains for interference figure work are those that show slightly lower birefringence near the center than along the margins, suggesting that the grain is oriented close to one of the optic axes. Once such a grain is identified, center it carefully and switch to conoscopic observation.
In BH250-41, these well-oriented olivine grains produce an interference figure in which the isogyre appears as a straight dark band rather than the curved, concave–convex isogyres seen in many other biaxial minerals. This is because forsterite has a very large 2V, close to 90°, so the optic axes are widely separated. As a result, the figure does not show the familiar curved brushes of a small-2V biaxial figure. Instead, students will observe a straight isogyre sweeping across the field. This is a key teaching point: the absence of strongly curved isogyres is itself diagnostic of a mineral with a very large optic axial angle.
A common point of confusion is that students may interpret this straight isogyre as an off-centered uniaxial figure. However, careful observation resolves this. In olivine, the isogyre rotates or “spins” around the optic axis as the stage is turned, reflecting its biaxial nature and large 2V. In contrast, in a uniaxial mineral, an off-centered isogyre behaves differently—both ends of the isogyre move up and down symmetrically during stage rotation rather than rotating around a point. Recognizing this difference in movement is critical for distinguishing between biaxial olivine and uniaxial minerals such as calcite.
To determine optic behavior more fully, insert the sensitive tint plate and rotate the stage. In this figure, you do not have a clear concave and convex side of the isogyre as you would in a small-2V biaxial figure. Instead, interpretation comes from watching how the addition and subtraction colors change as the stage rotates. In BH250-41, students should observe blue addition colors and yellow subtraction colors shifting position during rotation. The movement of these colors relative to the isogyre helps establish the vibration directions and reinforces the biaxial character of the mineral.
A useful step-by-step approach is as follows. First, locate a suitable olivine grain in XPL, preferably one with a slightly subdued center. Second, center the grain precisely. Third, switch to conoscopic light and confirm the presence of a straight isogyre. Fourth, rotate the stage slowly and observe how the isogyre moves—specifically noting its rotational behavior rather than simple up-and-down motion. Fifth, insert the sensitive tint plate and track where blue addition and yellow subtraction develop and how these colors shift during rotation. This sequence guides students from grain selection to confident optical interpretation.
BH250-41 is particularly valuable because it teaches students how olivine behaves in a natural ultramafic rock rather than in an idealized section. It also provides an excellent basis for comparison with other BH250 samples containing olivine, including BH250-82 (Hawaiian basalt), BH250-112 (Dish Hill xenolith), BH250-136 (Hawaiian xenolith), BH250-227, and Bh250-103b (Alta Stock marbles) where olivine is essentially pure forsterite (Mg₂SiO₄). Comparing these samples allows students to recognize consistent optical behavior across diverse geologic settings.
Comparison with other minerals further strengthens interpretation. In calcite, a uniaxial mineral, the optic axis figure is centered with a black cross and concentric isochromes, and off-centered figures show symmetric motion of the isogyres. In talc, a biaxial mineral with small 2V, students observe curved isogyres and can use concave–convex geometry to determine optic sign. In forsterite olivine from BH250-41, however, the very large 2V produces a fundamentally different figure: a straight isogyre, widely separated optic axes, and diagnostic rotational behavior.
BH250-41 is an excellent sample for learning the optical behavior of forsteritic olivine in thin section. In this dunite, several olivine grains are cut nearly perpendicular to an optic axis and produce a distinctive interference figure. Begin by scanning the thin section in plane-polarized light (PPL) and crossed polars (XPL) for grains that are clean, inclusion-poor, and relatively broad in view. In XPL, the best grains for interference figure work are those that show slightly lower birefringence near the center than along the margins, suggesting that the grain is oriented close to one of the optic axes. Once such a grain is identified, center it carefully and switch to conoscopic observation.
In BH250-41, these well-oriented olivine grains produce an interference figure in which the isogyre appears as a straight dark band rather than the curved, concave–convex isogyres seen in many other biaxial minerals. This is because forsterite has a very large 2V, close to 90°, so the optic axes are widely separated. As a result, the figure does not show the familiar curved brushes of a small-2V biaxial figure. Instead, students will observe a straight isogyre sweeping across the field. This is a key teaching point: the absence of strongly curved isogyres is itself diagnostic of a mineral with a very large optic axial angle.
A common point of confusion is that students may interpret this straight isogyre as an off-centered uniaxial figure. However, careful observation resolves this. In olivine, the isogyre rotates or “spins” around the optic axis as the stage is turned, reflecting its biaxial nature and large 2V. In contrast, in a uniaxial mineral, an off-centered isogyre behaves differently—both ends of the isogyre move up and down symmetrically during stage rotation rather than rotating around a point. Recognizing this difference in movement is critical for distinguishing between biaxial olivine and uniaxial minerals such as calcite.
To determine optic behavior more fully, insert the sensitive tint plate and rotate the stage. In this figure, you do not have a clear concave and convex side of the isogyre as you would in a small-2V biaxial figure. Instead, interpretation comes from watching how the addition and subtraction colors change as the stage rotates. In BH250-41, students should observe blue addition colors and yellow subtraction colors shifting position during rotation. The movement of these colors relative to the isogyre helps establish the vibration directions and reinforces the biaxial character of the mineral.
A useful step-by-step approach is as follows. First, locate a suitable olivine grain in XPL, preferably one with a slightly subdued center. Second, center the grain precisely. Third, switch to conoscopic light and confirm the presence of a straight isogyre. Fourth, rotate the stage slowly and observe how the isogyre moves—specifically noting its rotational behavior rather than simple up-and-down motion. Fifth, insert the sensitive tint plate and track where blue addition and yellow subtraction develop and how these colors shift during rotation. This sequence guides students from grain selection to confident optical interpretation.
BH250-41 is particularly valuable because it teaches students how olivine behaves in a natural ultramafic rock rather than in an idealized section. It also provides an excellent basis for comparison with other BH250 samples containing olivine, including BH250-82 (Hawaiian basalt), BH250-112 (Dish Hill xenolith), BH250-136 (Hawaiian xenolith), BH250-227, and Bh250-103b (Alta Stock marbles) where olivine is essentially pure forsterite (Mg₂SiO₄). Comparing these samples allows students to recognize consistent optical behavior across diverse geologic settings.
Comparison with other minerals further strengthens interpretation. In calcite, a uniaxial mineral, the optic axis figure is centered with a black cross and concentric isochromes, and off-centered figures show symmetric motion of the isogyres. In talc, a biaxial mineral with small 2V, students observe curved isogyres and can use concave–convex geometry to determine optic sign. In forsterite olivine from BH250-41, however, the very large 2V produces a fundamentally different figure: a straight isogyre, widely separated optic axes, and diagnostic rotational behavior.
Relation
Collection
Citation
“BH250-41,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/377.