Uniaxial stress was applied to the single crystals of Cr3B7O13Cl [Cr-Cl] boracite along {110}cub directions by a special rod constructed in the lab. and adapted to low temp. polarized light microscopy. Evidence for a stress-induced 1st-order phase transition from the cubic .hivin.43m or the tetragonal .hivin.42m phase to the orthorhombic mm2 phase was obtained by in situ observations of the ferroelastic/ferroelec. domains. The domain structure and the birefringence were analyzed and correlated to the crystal symmetry of the initial and the induced phase. For a mech. stress below the threshold strength, the piezo-optical effect was disclosed, leading to a linear and reversible birefringence, e.g. Dn = 4.8 * 10-4 under a stress of .apprx.22 * 107 N/m2 (22.4 * 102 kg/cm2) at 285 K. The piezo-optical coeff. p44 in the .hivin.43m and .hivin.42m phases was found to be 1.5 * 10-12 m2/N (T = 264 K). In the spontaneous mm2 phase the application of a uniaxial stress gave rise to ferroelastic poling (mech. detwinning) of the orthorhombic lamellar polydomain state, hence to ferroelec. poling due to fully ferroelastic/fully ferroelec. coupling of the domains. [on SciFinder (R)]