The accurate lattice parameters and the extremely small lattice strains of Fe I boracite, Fe3B7O13I, have been successfully measured over the paraelec. and the ferroelec. regions by using special x-ray diffraction methods. The spontaneous strains in the ferroelec. state could be sepd. into 2 parts, a linear strain (pure shear) and quadratic ones (electrostrictions). It is a conspicuous feature of the electromech. properties of Fe2B7O13I that the electrostrictions prevail over the linear strain induced through a torsional piezoelec. const. b36 of the paraelec. phase. The electrostrictive consts. are approx. 2 orders of magnitude larger than those of KH2PO4. Another phase transformation at a lower temp. was disclosed by the present diffractometry. At this temp., a special rhombohedral lattice is formed, and the piezo-elec. const. b36 vanishes.