End_Page 549------------------------------

The composition of common clinopyroxenes may be defined by measurement of the optical properties 2V_z and ß. The mole-per cent composition thus obtained differs from that obtained by chemical analysis and for some varieties there is no reliable correlation between the optical properties and the chemical composition. For a diopside from Oka Complex, Quebec, the optical properties (2V_z = 56°, ß = 1.685 ± .001) gave 48.1:41.9:10:0 for the Ca:Mg:Fe ratio, using the best curves available (Hess 1949), and the chemical analysis revealed the ratio 49.4:45.4:5.2.

We have applied optical properties to the determination of Ca:Mg:Fe ratio in various clinopyroxenes and we have observed the following degree of error by referring the composition determined by optical properties with that determined by chemical analysis

1. Diopsides-hedenbergites (mean value of error for 17 specimens) Mg:8.2 Fe:5.8 Ca:4.8

2. Augites (mean value of error for 7 sepcimens) Mg:3.6 Fe:4.5 Ca:3.6

3. Ferro-augites (mean value of error for 7 specimens) Mg:6.0 Fe:6.1 Ca:3.4

It may be possible to obtain a better definition of that composition by exact intensity measurements of X-ray powder diagrams. Several measures on a group of six clinopyroxenes reveal this possibility.

End_of_Article - Last_Page 550------------