Determining Sign and 2V from an Optic Axis for a Biaxial Mineral

Determining the optic sign from an principal axis figure can often be simpler than trying to hunt out a Bxa. to hunt out an OA figure, search for a grain displaying zero or very low retardation. Obtain an interference figure and rotate the stage. It should be an OA figure, showing one centered or nearly centered isogyre. If you see two isogyres that stay within the sector of view once you rotate the stage, you're watching a Bxa for a mineral with low 2V. Rotate the stage therefore the isogyre (or the foremost nearly centered if there are two) is concave to the northeast. 

Note that the isogyre rotates within the opposite sense from the stage. Insert the full-wave accessory plate. If the retardation increases on the concave side of the isogyre (and decreases on the convex side), the mineral is positive. In minerals with low to moderate retardation, it's often necessary only to seem for yellow and blue.Blue indicates a rise and yellow a decrease in retardation. the increase or decrease of retardation are becoming to be opposite if the mineral is negative.You can estimate 2V by noting the curvature of the isogyre. If 2V could also be a smaller amount than to the isogyre will seem to form a bend. If 90° 2V is it'll be straight. For other values, it'll have curvature.