BH250-41
Title
Subject
Description
Minor Minerals: serpentine, magnetite
Texture: equigranular olivine
BH250-41 is a dunite from Balsam, 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 nearly 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.
On the Olivine–Orthopyroxene–Clinopyroxene (Ol–Opx–Cpx) ternary diagram used for ultramafic igneous rock classification, dunite plots near the olivine apex because it contains more than 90% olivine, with only minor pyroxene if present. BH250-41 would therefore plot very close to the Ol corner of the diagram, firmly within the dunite field. This position reflects its highly magnesium-rich, mantle-derived composition and distinguishes it from harzburgite, lherzolite, and pyroxenite fields that contain greater proportions of pyroxenes.
Optically, olivine exhibits strong birefringence, producing bright second-order rainbow-like interference colors. It is biaxial with a 2V angle of approximately 90°, and in favorable orientations may appear nearly optically neutral. This sample provides excellent teaching examples for selecting grains that produce an optic-axis interference figure and for demonstrating the difference between biaxial and uniaxial behavior under the microscope.
BH250-41 also shows evidence in some thin sections of later metamorphic or alteration reactions, where primary olivine has begun to break down along fractures or grain margins. Common reactions include hydration to Serpentine or talc-bearing assemblages, recording fluid infiltration after crystallization. These secondary textures provide an excellent contrast between pristine mantle olivine and later alteration during tectonic uplift and weathering. A simplified serpentinization reaction is:
3(MgFe)2SiO4 (forsterite) + 2H2O + 2O2 → 2Mg3Si2O5(OH)4 (serpentine) + Fe3O4 (magnetite)