Summary: Olivine composition can be rapidly and accurately obtained by using standard (Straumanis-mounting) 57·3 mm X-ray powder cameras. A determinative curve based on analysed natural olivines has been constructed by plotting the mol. % Fo against the spacing of back-reflection line 174. The regression equation for the composition range Fo₁₀₀ to Fo₃₀ is

Fo (mol. %) = 4151·46−3976·45 d₁₇₄

using Fe-Kα₁ radiation. The fayalite end (Fo₃₀ to Fo₀) of the determinative curve shows a relatively sharp break from a straight-line relationship of d-spacing versus composition. Practical applications of the determinative curve are briefly presented.