BH250-220
Title
BH250-220
Subject
Zircon Cut Perpendicular to C-axis
Description
Major Mineral: zircon
BH250-220 — Zircon Cut Perpendicular to the c-axis (Optic Axis Figure Teaching Sample)
BH250-220 is a specialized optical mineralogy teaching sample designed to help students understand how crystallographic orientation influences interference figures observed under the petrographic microscope. This thin section contains unusually large zircon grains that were intentionally cut perpendicular to the crystallographic c-axis, allowing students to observe an optic axis figure, a feature that can be difficult to obtain in zircon because most zircon grains in standard thin sections are too small for detailed optical work.
Zircon (ZrSiO₄) is a tetragonal mineral and is classified as a uniaxial positive mineral. While students commonly learn uniaxial optics using quartz, which is abundant and relatively easy to observe, other uniaxial positive minerals are less commonly available in teaching collections. Quartz is often the most familiar example, while Corundum (BH250-221a) provides another less common example. Zircon offers an excellent additional teaching specimen because it allows students to compare optical behavior across different uniaxial minerals.
Under crossed polarized light using conoscopic illumination, the section displays a slightly fuzzy optic axis figure, which is not unusual for zircon due to its very high relief, strong birefringence, and occasional internal imperfections. Despite the slight fuzziness, the figure clearly demonstrates the diagnostic features of a uniaxial mineral. When a mica (sensitive tint) plate is inserted, students can observe the addition and subtraction quadrants, allowing them to determine the mineral’s optical sign. This exercise helps reinforce the distinction between uniaxial positive and uniaxial negative minerals and gives students practical experience interpreting interference figures rather than simply memorizing diagrams from a textbook.
This sample is particularly valuable because large zircon crystals suitable for this type of thin section are uncommon. Most zircon grains encountered in igneous, metamorphic, and sedimentary rocks are accessory minerals that occur as very small crystals, often too small to generate clear interference figures. BH250-220 therefore provides a rare opportunity for students to directly observe zircon’s optical properties and better understand the relationship between crystal structure, crystallographic orientation, and optical mineralogy.
Beyond its instructional value in optical mineralogy, zircon is also one of the most important minerals in geoscience because of its exceptional durability and ability to preserve geologic time through U-Pb dating. Its resistance to chemical weathering and metamorphism allows zircon crystals to survive multiple geologic cycles, making them invaluable tools for reconstructing Earth history. This sample connects optical mineralogy with broader applications in petrology, geochronology, and sedimentary provenance studies
BH250-220 — Zircon Cut Perpendicular to the c-axis (Optic Axis Figure Teaching Sample)
BH250-220 is a specialized optical mineralogy teaching sample designed to help students understand how crystallographic orientation influences interference figures observed under the petrographic microscope. This thin section contains unusually large zircon grains that were intentionally cut perpendicular to the crystallographic c-axis, allowing students to observe an optic axis figure, a feature that can be difficult to obtain in zircon because most zircon grains in standard thin sections are too small for detailed optical work.
Zircon (ZrSiO₄) is a tetragonal mineral and is classified as a uniaxial positive mineral. While students commonly learn uniaxial optics using quartz, which is abundant and relatively easy to observe, other uniaxial positive minerals are less commonly available in teaching collections. Quartz is often the most familiar example, while Corundum (BH250-221a) provides another less common example. Zircon offers an excellent additional teaching specimen because it allows students to compare optical behavior across different uniaxial minerals.
Under crossed polarized light using conoscopic illumination, the section displays a slightly fuzzy optic axis figure, which is not unusual for zircon due to its very high relief, strong birefringence, and occasional internal imperfections. Despite the slight fuzziness, the figure clearly demonstrates the diagnostic features of a uniaxial mineral. When a mica (sensitive tint) plate is inserted, students can observe the addition and subtraction quadrants, allowing them to determine the mineral’s optical sign. This exercise helps reinforce the distinction between uniaxial positive and uniaxial negative minerals and gives students practical experience interpreting interference figures rather than simply memorizing diagrams from a textbook.
This sample is particularly valuable because large zircon crystals suitable for this type of thin section are uncommon. Most zircon grains encountered in igneous, metamorphic, and sedimentary rocks are accessory minerals that occur as very small crystals, often too small to generate clear interference figures. BH250-220 therefore provides a rare opportunity for students to directly observe zircon’s optical properties and better understand the relationship between crystal structure, crystallographic orientation, and optical mineralogy.
Beyond its instructional value in optical mineralogy, zircon is also one of the most important minerals in geoscience because of its exceptional durability and ability to preserve geologic time through U-Pb dating. Its resistance to chemical weathering and metamorphism allows zircon crystals to survive multiple geologic cycles, making them invaluable tools for reconstructing Earth history. This sample connects optical mineralogy with broader applications in petrology, geochronology, and sedimentary provenance studies
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 220. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-220,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/290.