Egg Your Optical Crystallography Students On Optical crystallography is easy to understand if a 3-dimensional picture of the optical indicatrix can he visualized hy the student. An easy to make and inexpensive tool to aid in this visualization is an indicatrix made from a chicken egg. The egg serves a8 an approximate model for the uniaxial or hiaxial indicatrix. The model is approximate since the egg possesses C,, point group symmetry, whereas the uniaxial and hiaxial indicatrices areD-h andDzh, respectively. The egg model has been successfully used in undergraduate laboratory classes in the following manner. At the time the students are introduced to the biaxial indicatrix, they are requested to hnng a hollow chicken egg to their laboratory class. Extra large white eggs are best. They are assigned to mark the egg with the following: principal vibration directions ( X , Y, Z ) , optic axes (OA), and traces of the circular sections (CS).While the students are working on the day's laboratory exercise, the instructor quizzes the students individually on their understanding of the biaxial indicatrix. With the labeled model of the indicatrix in hand, have each student (1) identify the principal sections and the optic normal, (2) lahel the vibration directions and their measurable refractive indices for several ray paths that you select, and (3) determine the optic sign of their indicatrix. The answer to (3) can he either positive (+) or negative (-1 because the optic axes could he located anywhere in the optic plane. If the student happens to draw the optic axes a t 45' from X and Z, the ambiguity of labelline the indicatrix (+I or (-)will he illustrated. This method of one-on-one auizzine clearlv~inooints which students
CS marks the two circular sections and OA the optic axes. Note that the indicatrix is negative. The students can save their indicatrix models indefinitely and have them handy for future reference. In particular. they can use the model for visualizing the orientation of the indicatrix of a mineral in a grain mount or thin section. Bloss, F. Donald, "An Introduction to the Methods of Optical Crystallography," Holt, Rinehart and Winston, Ine., New York, 1961, p. 172.
Kathleen A. Affholter Depariment of Geological Sciences Virginia Polytechnic Institute and State University Blacksburg, V A 24061
,leh Rea1ionsn.p of ray palns 00 OX. 0 5 0 A . OT ORlolnew assoc sled Pr b legea u orill on a recl onstaher B ars'r i r ghl A no 0% cnocren egg m e ed w in pr nc Pal 5 orn1.m d rccl ons . X Y , 2l opbc axes OA,, clrcb ar sccr oni ICS. ana the Pr u legea alrecttuns assoc ate0 w m a random ray path like OR
196
Journal of Chemical Education
