BH250-41

BH250-41

Dunite

Major Mineral: forsterite, olivine
Minor Minerals: serpentine, magnetite
Texture: equigranular olivine

BH250-41 is a dunite from Balsam, North Carolina, composed almost entirely of forsterite-rich olivine. It is one of the most visually striking rocks, both in hand sample and especially in thin section. Under crossed polars, the olivine grains display vivid second-order interference colors. Well-oriented grains cut perpendicular to the c-axis yield excellent biaxial interference figures, with a 2V angle close to 90°, making the mineral appear optically neutral. In this orientation, the isogyre forms a straight line that rotates smoothly about the optic axis—an ideal demonstration for teaching optical mineralogy.

Optics: Exhibits strong birefringence, 2nd order rainbow-like interference colors, biaxial optical character with a 2V angle of approximately 90°, and appears optically neutral in certain orientations. Includes instructional videos demonstrating how to identify and select grains that produce an optic axis interference figure.

Reaction: BH250-41 shows evidence of the following reaction in some of the thin sections. 

3(MgFe)2​SiO4​ (forsterite) + 2H2​O + 2O2 ​→ 2Mg3​Si2​O5​(OH)4​ (serpentine) + Fe3​O4​ (magnetite)

Balsam, North Carolina, USA

Bereket Haileab

From the rock collection of Bereket Haileab. Sample BH250-41. Housed at Carleton College in Minnesota.

Thin section and hand sample

View on ArcGIS Online here


































Olivine interference figure


Olivine interference figure


TAS diagram of sample BH250-41.