[en] Design and operating principle of X-ray fluorescence spectrometer for rock analysis, installed on the ''Venera-13'' and ''Venera-14'' landing capsules are described. X-ray fluorescence spectrometer is composed of two units: detection unit, containing gas-discharge counters and radioisotopic alpha and X-ray sources, and multi-channel pulse analyzer. The technique for spectra measurement and preflight experiment preparation are considered. Data on the Venus rock composition obtained for the first time, enabled to develope motions about the character of the Venus rocks, their o origin and formation